You could use SysInternals free program SDelete which will clean your disk free space (i.e. it will wipe all traces of previously deleted files.
It is a command line utility which means you go to Start> Run> and type 'command' or 'cmd' (no quote marks) and press Enter.
This will bring up the black/white command window.
Just drag the extracted file 'SDelete.exe' onto the window and you should end up with a line something like this, depending on the location of your copy of SDelete:-
C:\Documents and Settings\User Name>"F:\My Downloads\All Downloads\Compressed\sdelete\sdelete.exe"
If, for example, you want to wipe the free space on your Drive E, with 3 passes, you would add (space)-p 3 -z E: so you end up with the line below:-
C:\Documents and Settings\User Name>"F:\My Downloads\All Downloads\Compressed\sdelete\
sdelete.exe" -p 3 -z e: (don't forget the colon after the drive letter)
Now press "enter" and it will start. It will take a while depending on how many passes you require it to make, but you'll end up with a nice sanitized disk with only the operating system on it (assuming you had already deleted everything except the operating system).
If you enter no drive letter (by missing out the " -z etc" bit) then it will just wipe Drive C with your specified number of overwrites.
SDelete works on Windows 95, 98, NT 4.0, Windows 2000, XP and Server 2003.
Sorry to be long-winded about the above but better too much info than too little - and I don't know if you know about command lines!