[im_not_1337]

I've seen this many times and it almost always is some sort of logger application. What is usually happening is something is being logged and the log grows and grows untill your hard drive is out of space.

I doubt it is any sort of malware. When did the problem start occurring? Did you install any new software or make any windows settings changes around that time? Do you use peer guardian by chance? Sometimes its log has been known to grow to many gigs. Try to look through your files with nothing hidden and look for files that are abnormally large or seem to grow.