What program was it, what antivirus do you have already (you /do/ have AV already don't you)?. Sounds like something is causing Explorer.exe to crash. Hit F8 repeatedly when you switch the PC on, before you see the boot logo, select Safe Mode With Networking, if you can access anything do an AV update and a full scan, download a couple of anti-spyware apps and install/update/scan. Try Spybot & SuperAntispyware to be going on with; click here
I am assuming you have XP, as you mention system restore.