If you are a normal non power user, look in \"my documents\".
Otherwise, check folders where you download to often, uninstall old games you don't play anymore (D3 actually uses around 1.5+ GB full install + levels if you let it go). And uninstall anything that hogs space in program files that you don't use.
Run chkdsk /f in command prompt to check the drive for errors, sometimes disk space is lost that way (even on NTFS, if it finds something, in folder options ---> view tab, uncheck \"hide protected operating system files\", set it to show hidden and system files, and the recovered files will be listed in a chkdsk.000 folder or something like that, 99.999% of the time these files are useless old data you don't need, delete them).
Run disk cleanup to clean out the windows temp folders, also in disk cleanup, click the options tab and then the system restore button, system restore can hog huge amouts of HDD space after a long time. To prevent system restore from doing that again, go into system properties --> system restore tab, and turn down the amount of space allowed, or disable it outright.
If you never use it, disable hibernation mode to free up a chunk of HDD space equal to the size of your system RAM.
And your final option if all other methods fail: 300 GB hard drives are pretty cheap lately.
