I'm going to suggest a simple way of doubling your hard drive space, at least if your computer was originally set up by a major manufacturer... I'm afraid that unless your computer isn't in this category your only option might be to go and buy another hard drive...sorry.
The trick is possible because most manufacturers split your hard drive into two main partitions, allocating the operating system to Drive C and data to Drive D. Maybe this is to save your data such as photographs if your hard disk crashes? Alas, many hard disk crashes will lose both C and D, and if only C is lost it might cost a small fortune for the stuff on D to be recovered so I'd advocate backing up photos on an external drive, a large memory stick or DVDs.
It's a fact that most computer users haven't a clue about Drive D. Most of the computers that pass through my hands have a pretty full Drive C and a virtually empty Drive D. So how about just merging the two and benefiting from a larger Drive C?
Firstly you need to determine your operating system. If you're using Vista or later then all you need are built in tools. Just a handful of mouse clicks and you've doubled the size of Drive C. Do not read further if you're not computer literate, because if you bash on with limited experience you might come a cropper. Read on if you're familiar with computers and want to give a friend's computer (or a customer's computer; if you're into that stuff) an extra lease of life. For earlier systems like XP you'll need a proprietary application.
When Drive C gets full and it's already been cleaned up just add Drive D to it and you're good for another couple of years.
The next step is to see exactly what's on Drive D. If it's been used for storing stuff you'll need to back this up, but I'll describe a typical example I fixed today. Drive D was almost empty. It had some strange backed up information residing in two areas, one area was dated 2011 and the other 2012 (it's now well into 2015) so whatever was backed up was way out of date and possibly was the work of a now defunct program. If you're unsure just copy the data and keep it safe. To be certain you get it all you'll need to view hidden files and save these as well.
If you right click My Computer (or its newer name) and select Manage, then Disk Management, you should see the hard disks. Windows normally grabs a smallish area for admin called "System Reserved" or suchlike and it won't have a drive letter... Leave this alone. Drive C will have a name such as "System" and have appended descriptions such as "Boot", "Healthy", "Primary Partition" etc. Note an important point here. Some computers will have a different letter for their primary partition. There was a time when the installation of a new operating system tripped over devices such as camera card slots. For some odd reason one of these may have been given the letter "C" forcing the operating system to be allocated "H" or something different. This is fairly rare but did happen sometimes... just a warning and I'll assume your computer is normal and uses "C" for the operating system.
Some manufacturers use an area of the hard drive for restoring a crashed operating system and this is often given a drive letter. Unless you'd like to pinch this area to add to C this isn't the main object. If you'd like to bag this area just treat it like D.
If you select and right click the area labelled "C" or "D" you'll see, amongst others, two options "Extend Volume" and "Shrink Volume". The latter will be valid but the former may be greyed out (ie. not valid)
The rule for extending a volume is that there must be unallocated space immediately next to it, so if you applied Shrink Volume to D you'll end up with unallocated space but this will be after Drive D not Drive C so you cannot use this for extending C.
Right, we're finished with preliminaries. Select "D" and just apply Delete, which is further down the list. So, having established that D has nothing of interest stored on it, delete D.
This will provide unallocated space next to C and this is what we're going to use to add to C. If you right click C you'll now see that not only Shrink is possible, but Extend Volume will no longer be greyed out. Just apply Extend, letting it use the default space and there you go... Drive C will be doubled in size. If C had been pretty full the computer will have been rattling away trying to make best use of what little space had been available, but now with acres of new free space it will be a lot quieter and the computer will be a lot faster.
Do you want your computer to go like a rocket? This is a simple step. Most proprietary computers when new will have been pretty quick, but as they get older they get full of rubbish which slows them down. Getting rid of the rubbish is an art but there's a simpler way to regain most of the original speed. The penalty is to stop Windows using its bells and whistles. All those daft features added by software boffins using mainframe computers that are (speedwise) streets ahead of normal everyday laptops. I counted twenty such bells and whistles on my Windows 7 computer. Stuff like "Enable transparent glass". I contend that transparent glass is worth kicking into touch if it meant a 5 minute wait while your computer turns off. A slight exaggeration of course because you can't blame transparent glass for everything... add the other 19 frills and you probably can though.
Just right click My Computer (or whatever fancy name Microsoft has changed this to) and select Properties. Click on Advanced Settings and you'll see Performance. Click on Settings and select "Adjust for best performance" and there you go... unless you really can't do without Transparent Glass in which case just tick this (or any other frill you can't do without) but the more bells and whistles you add the bigger the ball at the end of the chain....
Selecting programs, booting up and turning off should now be almost as good as when the computer was born.
While you're at it why not get rid of the rubbish installed by the manufacturer? Sony, for example is one of the offenders. I call the stuff they install "Scrap". "S" for Sony.... but Acer, HP and all the rest do much the same. I have a nagging feeling that Microsoft is now joining in with their Windows 10 (a tip here is to do a custom install). Also offending are printer suppliers. Get rid of anything dubious by uninstalling via Control Panel. Another tip is to use your eyes when adding new programs. Keep an eye on ticks in boxes otherwise you'll be inadvertently adding McAfee or all sorts of stuff you can do without. When you need a new program, say Google Chrome, make sure you get this from the real site not a look-alike which may give you Chrome but might also supply you with Adware or even a virus... Always think hard about clicking "Download" because the thing you wanted might have a small box to click and that larger box is the one paying the supplier that installs something really weird. I could go on but I'll end here.. but first take a look at one of the very first hard drives below.