I don't know what numbers you are using. I came into this thread late (#42) so maybe I missed something.
Correction on my percentage. It is 73%, not 77%.
The percentages posted in #15 add up to 73%. 73% of 16.3Gb is 11.89GB. I am just using the OP's numbers.
The only other thing I can think of is hidden files may not be included for whatever reason.
You can change the settings to show all files. Look at the link below to learn how.
Also, explore your Windows folder and see if you have a temp folder in there. If so, how much data is in there?
Did you ever try shutting off System restore and then turning back on. That will remove any old restore files. You can also remove the Windows update files.
Was this computer upgraded from an older version of Windows to WinXP? Could be there are extra files you don't really need there too.
30% of 16.3GB is 4.89GB. I am running WinXP pro SP3 and my size on the disk is 2.58GB. Windows shouldn't use as much space as It appears you are using.
If you have an installation CD for WinXP then you don't need the I386 files on there either.