Go to C:\windows.
Look for the folder servicepackfiles.
Delete the folder. If you are worried about needing this at some point just back up to a DVD first.
Look for some blue folders labeled $NtUninstall or similar.
If your computer has been running stable for the last few weeks and you don't plan on uninstalling your service pack, then delete all these folders. *Hint* If you don't know what service pack is, then you should delete these folders.
Goto the system32 folder.
Look for the folder drvstore (should be blue).
If you don't have any problems with drivers, then delete it.
There are also many guides to gaining space on drive C:
http://helpful.knobs-dials.com/index.php/DLLCachehttp://www.graphixanstuff.com/Forum/index.php?showforum=68*Note* If you ever plan on reinstalling windows, I recommend you use MicroXP.
Buy a windows licence. Install MicroXP. Copy the licence into the MicroXP installation.
Now you will have a windows folder of about 250MB (400MB with all drivers and software installed).