[Raptor]

If you re-install windows without formatting the hard drive it only overwrites the files it is installing, it doesn't go through and erase stuff that shouldn't be there or cookie files or otherwise. Formatting is the right way to reinstall in the case you are describing. Back up whatever files you consider important and if you want to make sure there is no question, go to a command prompt and type format C: let it finish and you will have a freshened disk ready to install windows like it was never there.