Hard to know what to say when you cant actually see the pc for yourself but it sounds fcked chap, without getting all technical with you do you have a windows xp/vista disc? if you do id surgest the easiest method here rather than to try and sort each issue individually(if thats possible) is to back up everything onto cd/dvd or external hard drive then just re install windows and put everything back on.
Might sound like a big deal for someone who isnt "into" pc's but honestly sometimes you have to just say "sod it" and reinstall windows, as i said above sometimes its the much easier option and i know for a fact once you have a clean install of windows it should run fine