Go to Start > My Computer > right click on C drive > properties. That will show you how much HDD space you have left.

If you're low on disk space, try using something like CCleaner(freeware). You can also do a disk cleanup (All Programs > Accessories > System Tools) & a defrag won't hurt. If you're desperately low on space, you might have to think about getting rid of some files from your computer or getting another HDD.