Someone said :
Apart from re-installing windows you would need to get a program that does a better cleaning job that the windows remove software program, but i think you still have to tackle each program one at a time and you cant just delete everything but the OS in one swoop.
Even if you do reinstall windows the files can still be restored with software, best way to do it is called a low level format of the hard drive this ensures you get rid of everything then reinstall windows after the low level format.
perhaps something like this might help if this is what you are looking to do, http://hddguru.com/software/HDD-LLF-Low-Level-Format-Tool/
there is other low level formatting tools available just do a google on it.
Hope this helps.