There are chances that your C drive may have many temporary files which is created during program installation (sometimes, the clean up application on the installer may not remove those temporary files). Check the folder "C:\Documents and Settings\<username>\Local Settings\Temp" and see whether you have chunks of files there.. You may just remove those files in order to free up more space.
Also check for the temporary internet files folder at "C:\Documents and Settings\<username>\Local Settings\Temporary Internet Files" and remove the items which you no longer need in your internet cache (If your settings are set to Hide protected operating system files, then you may not see this folder in your windows explorer window. In that case, just type the folder name in the address bar and you can access the contents inside the folder).
Anyway, if you just want to alter the space available on your partitions, you may use many useful tools available for that.