How can I fix multiple hangs and non working buttons?

I cannot get into anything via the start menu (eg. control panel) as everything "hangs", also most desktop icons do not work (IE does, obviously) and when I attempt to use desktop icons, the start bar disappears never to return. I constantly have to shut down manually. Started with shut down menu taking a very long time to appear and has got progressively worse until almost all of PC is unusable. Have had mega problems with home page hijackers (esp CWS) and I keep getting "Only the best" pop-ups. Have tried umpteen software products to cure this, largely to no avail. Have also tried system restore which does not seem to make any difference to the problem. I'm using Windows XP Home, SP2.

Would be more than grateful for any suggestions.

Thanks

Harry