While you did not state what OS you are using, I will assume Win2000. Similar comments apply to Win NT, but the files are in different locations.
Since you are experiencing data growth, I would suspect that either users are storing data under their Documents and Settings areas, or you have log files that keep getting added to.
I would check to see if you have a memory dump file (as big as the Ram in the system) that was written because you had a system crash (Blue Screen of Death!) at some point. This can be deleted.
If this is a server, remove any extra user accounts except those for the the administrator. These accounts can be quite large because of the documents stored under each user, so be careful, and make sure you back it all up in case you need to restore anything.
I would for sure delete all the temporary Internet files (under each user listed under the documents and settings directory, username, local settings)to include all the funny numbered directories listed under the folder called content.IE?. (note, the system will re-create these numbered folders, so you can say yes when the system tells you these are system folders, etc.)
NOTE: You will have to make sure the directory view is set to display hidden directories in order to even see these files under the Local Settings directory (In Win Explorer, go to Tools, folder options, view, show hidden folders).
Next, in all users Temp directories, I would clean out all files with ~??????.xxx as a name, as these are old temp files, no longer needed. You can also clean out the files ending in .tmp, especially if they are 0 bytes in size. Some programs tend to be very sloppy about cleaning out their temp files, and this can really eat up disk space.
I do not know how many applications you have on this system, but you could uninstall these applications, then re-install them, but do not use the default location, use another drive location instead. While this will not save any space under the OS, it will move all the rest of the application files to the other drive, (which probably now reside under Programs and Files) and this can be a very large amount of drive space for applications like MS Office (hundreds of MB).
Another tool to add usable space is to compress directories on the hard drive, but I would be very careful here. You can probably recover a lot of space by compressing the Program Files directory, but you will incur a performance hit on the system doing this, which should be ok if this is a fast system. Another area to compress is the Documents and Settings directory (assuming you could not delete most of these users), as this area easily compresses and the only piece of it being used at any time is from the one user currently logged on.
If this system is also a print server, then make sure the print spooler area is pointed to another drive. This is very important, especially since you have so little space left. A user printing a large file or many jobs at once could take all the free space on the drive and crash the server with a 100% full drive. Not fun, as you probably would not be able to boot the system at that point. Different problem, but one to worry about for sure.
HTH
David
