available drive space

You didn't mention whether or not you had a second harddisk or partition available to move software to. If you do, the easiest thing to do is uninstall a software package from your C: drive and re-install it on the D: drive.
If you do not have a second drive available, there are a few alternatives:
1) Use removeable media. A Zip, Superdrive, or CD-R/W drive can be used to store large amounts of data. Even floppies can be used to make backups of some data that can hen be removed from your harddisk.
2) If there is anything in the Recycle Bin, delete it.
3) Remove the contents of all temporary directories. These folders are usually called "TEMP" or "TMP" and may be in the root directory or under the Windows folder.
4) Delete temporary files. Search for files that end in ".tmp" or begin (or end) with a tilde (~).
5) If the system is used on the Internet, remove the contents of the Temporary Internet Files" folder. This is easiest accomplished in your browser. If you use IE, the control to empty this folder is under "Interne Options."
Hope these ideas have been helpful.

Hi - Kind of guessing on this, but it sounds like your backup software needs some space to do file compression prior to putting files on the backup media (PKZIP does this, probably others). It is likely it hit a large file (or files) that when compressed exceed the free space on the temp directory (wherever that is, C:\TEMP?).

Answer #1 indicates some solutions to that, partition, clean disk, move applications, etc.

If you have a second disk (possibly including a Zip, Jaz, etc.), you canplace \TEMP on that disk during the backup. Depending on size of media, may not resolve (if you use a 100mb Zip and need > 100mb, no help).

Try locating file that is so large it compresses to > 15mb (check the backup for last file (if possible) search disk from that file/directory path looking for large files).

Might also specify the OS and the Backup Software, there may be a way to redirect the temp/work area in the setup.

If you have compression on, disable it and see if that resolved (this will take at least 2x the media). If backup w/o compression works, above is probably the problem / solution. If not, it's something else. If you don't have sufficient media, you can 1) buy more media; or 2) test by writing volume 2, 3, 4,... on volume 1 (be sure to disable or abort the verify). For 2) YOU DON'T HAVE A BACKUP so don't delete / partition / etc. anything.