Sorry, my Chinese typing is very poor. Anyway, here are my opinions:
If your Norton run 到一半就"輕機", that could mean system files conflict. I would suggest you disable Windows system restore feature first (Win ME/XP only). Next restart Windows in safe mode. Then scan and clean virus again. Finally, restart Windows normally and enable system restore. Of course, your virus signature file must be up-to-date.
If problem still persists afterwards, then the last resort would be to format harddisk and re-install Windows like you said. But you should have all the drivers and installation softwares ready before you start. You should also backup all the important data and document files first.
Windows 98, 2000 or XP? It depends on the config or performance of your PC as well as applications you use. If your PC is very old one and doesn't have enough memory or disk space, then 98 is the way to go. Otherwise, I would recommend XP (precisely XP Pro if you can afford).