You didn't give any detail about your OS and installations. If you're using XP, disable hibernation. The hibernation file takes up as much space as the amount of RAM in your system and is in the root directory of your C: drive as a hidden system file named "hiberfil.sys".
However, do not try to delete the file itself. Disable hibernation from Desktop properties -> Screen Saver -> Power -> Hibernation tab. If you untick the "Enable hibernation" box, the hiberfil.sys file will go away by itself.
If you installed WinXP service pack, it saves backup files in case you want to uninstall the service pack. Most of us simply format C: and reinstall everything from time to time. If you're like that, just delete the service pack backup files. They are located in a hidden folder inside C:\Windows\. You can free up hundreds of MBs. The folder name is $NtServicePackUninstall$.
Some software installation packages, especially self-extracting packages, extract the package to a temporary folder in C:, but do not delete them after installation. You can also delete those. They are usually located somewhere inside Documents and Settings\Username\.
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