You can do the suggestions above and it MAY fix it, but I worked in IT for years fixing the darn things and they are never 100%.
My professional opinion is to organise all you personal files, photos, etc and back them up to DVD or another hard drive.
Then format the hard drive with a Windows re-install.
Sure you have to load up all the software again and it takes time, but most of the software may be outdated or irrelevant now, but you'll be better off for it.
It's cheaper than paying for a tech at $70+ p/hr too.