Not a good idea to move the essential Data Library folders located in C:\Users\Username as mentioned by nmsuk.

These include the 4 folders: Documents, Music, Pictures, and Videos. Moving those over to a different logical partition or physical drive, i.e.:
D:, E:, F:, etc. is very bad. kemical's suggestion of moving large files from those folders to backup folders with similar (but not identical!) names will definitely help. Ideally, you want your Windows to keep those 4 folders on the C: drive, but they need to be mostly empty. Files in your Music and Videos folders typically will take up the most room on your drive as they contain the largest individual file sizes.
You can also download the free
CCLEANER cleanup tool from
Link Removed to clean off the remaining stuff; but it's typically just a couple of Gigs or less worth of space you'll gain. The big offenders of disk usage space are your Apps (programs) as already mentioned, and your files contained in your video and music folders as I said. Videos made by your WMM app can take up Gigs and Gigs especially if you have incorporated audio files to use as soundtracks (music or voice narration) for those movies you made take up space quickly.
If you are using W7; most likely you have a Hard Drive from
100GB-500GB or thereabouts unless you have already upgraded your C: drive since you bought the computer to a larger drive. An example using kemical's formula on a 100GB drive would be you need
at least 20% free (I prefer to have 25% free) and that means you need to get that drive down to 80GB used--no more! before your Windows becomes stable. So, you telling us you have less than 1/2 Gig free that tells me that on a 100GB drive you have 99.5GB used or 95.9% of your drive used (0.045% free).
You need 20% free and you have 0.045% free; that's really really Bad! 
You already know this from our comments. Get those movies and other large apps no longer needed or used off your C: drive soon or your whole computer will crash! If you have a larger drive >100GB then it's not as severe; but Windows computers having less than 10% free space are on the verge of crashing regularly, or at the least hanging, freezing, or Blue Screening (BSOD).
If you cannot get your C: drive to at least 20% free, it's time to replace that C: drive with something of adequate size; a 1TB drive for example, which costs under $50 new on amazon or ebay for a desktop PC. Folks who do home movie making, or intensive graphics design such as CAD are always crashing their computers because they fill up their C: drive and never know why or understand the result of doing so!

If you are unable to replace the drive due to the huge hassle of reinstalling your Windows--you can take your computer to a local repair shop and have them transfer over your entire OS (W7) programs and data to the new larger hard drive for you and get you some space you sorely need. It would be cheaper than paying a Tech to recover your crashed computer due to no space left! Just sayin.
P.S. Hope you have all those Movies backed up to external media somewhere just in case...
Hope that helps.
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