Hard Drive - 0 bytes free

[Arra]

My computer keeps saying that my 465 gig harddrive is full when it's not. I really don't know what to do. Should I get a new harddrive? Or is it a problem with some other computer part? Or is it because of Vista? This is what happens every time I turn on my computer, if I click on C: drive properties: - It says about 250 GB are free. This is fine, what it should be. - A couple hours later, it says around 200GB are free - A couple hours later, it says around 100GB is free - This continues until it says 0 bytes are free. This usually takes about 5 hours after turning on the computer. The computer acts as though there really are 0 bytes free. I can't save anything, even text files, my browser won't load any web page images, and it keeps telling me to do a disk cleanup to save space. - At this point, if I delete a file, it will say the space is free. For example, if I delete something that's 1.5mb, it will say 1.5mb are free on the harddrive. But, after a minute or so, there are 0 bytes free again. - I restart my computer, and everything is fine again. 250 GB are free. - This process repeats. Additional information: - I usually keep my computer on almost all the time. However, since this started a few weeks ago, I've tried turning it off as often as possible. I need to sleep with it on during the night to drown out the sound of my dad snoring. Turning it off for hours at a time while at school doesn't seem to help. When I turn it back on, it's as bad as before. - This has happened before with this computer, about a year ago. I went on a trip for a week, and when I came back, the problem no longer existed. - I'm running Vista (is this the problem?) - I've scanned my computer for viruses multiple times with a couple different antiviruses, and nothing has been found. - Doing a disk cleanup doesn't work. All that does is delete some useless files, which temporarily makes the computer say a few bytes are available, but within a minute or seconds it's down to 0 bytes again. I really don't mind it that much. It isn't a huge issue, since restarting my computer every 5 hours is a minor inconvenience. I'm just worried that it will begin not to work even after restarting, because my computer is very important to me. I'm on it almost all day, almost everything I do involves it, and the few times it's been unusable in the past for whatever reason, it's been hell. When it's saying 0 bytes are free, it's basically unusable. Thanks for reading, and thanks in advance for any advice.