Have you gone to `My Computer/C:` and right clicked on `Windows`? it should be around the 8GB mark if it is similar to mine (XP Pro SP3).
I had a very large amount of `dross` in the Windows file from updates and RegScrubXP click here was recommended to me. Click on `RegScrupXP Finds Problems` and, when it has finished its scan, `Select All Problems` followed by `Fix Selected Problems`. It clears a lot of space - or at least it did for me.