If you are running Windows Xp have you got system restore switched on? This can potentially use a lot of disk space. Right click My Computer - Properties - System Restore and change the settings, if you are happy with the way your machine is running switch it off then back on this will remove all old restore points. This should help you free a little disk space.
Another way to reclaim some disk space is to delete all your old windows update uninstalls which are found in the windows directory in folders called $Nt... this is more a desperate measure but if your system is running OK then you can probably live without them but I have reclaimed a lot of disk space doing this in the past.