How can I fix my computer?

My computer runs on windows xp sp2 (i think, it won't let me check). I surfed the internet for a long time without any problems. I found out later that I had a whole bunch of viruses. So I did the logical thing, got an antivirus. The problem is that at least one of the viruses was imbeded in the operating system files, because now when I click on a shortcut on the desktop it will only go to the "open with..." screen. So I went to the control panel but it did the same thing when I clicked on anything in there. It will run files that are already saved like word document and such (if the've been saved prior.) but it doesn't seem to be able to open shortcuts. So I would like to know whats wrong with my computer and if there is anyway to fix it without reformatting the hard-drive, reloading the operating system, and basically starting over and deleting everything.