When the %SystemDrive% (commonly drive C:) starts to fill up "by attrition" the culprit is usually two things: temporary Internet files and files left in the C:\\TEMP directory.

Temporary Internet files are not always removed from the General > Temporary Internet Files tab under Properties. Files may need to be manually removed from %SystemRoot%\\Temporary Internet Files and %SystemRoot%\\Profiles\\[UserName]\\Temporary Internet Files, where [UserName] is the logged in NT user. (NOTE: removing these files is not encouraged by Microsoft; remove these files at your own risk).

More obscure zero-byte .TMP file may appear in (or under) the %SystemRoot% directory. Workstations with a Personal Web Server installation may also have .LOG files build up in the %SystemRoot%\\system32\\LogFiles\\W3SVC1 directory.