My computer has been so slow for some time since my Norton anti virus expired. I suspect i now have the virus. How do i clean it up?

The last time I just installed over the windows 98 and that didnt go so well. Does it really matter? Any suggestions would be greatly appreciated. Thank you…………:)

I want a free one that will clean my registry, delete needless files, defrag, etc.

My PC has been crashing and people tell me to just do a clean reboot of the OS but when I try I keep getting the message that the version on my PC is newer than the one on the CD. I know that I’ve downloaded updates, hence the version on the cd is old, but I don’t mind going back and updating it later. I just want to clean my system out and update it after i have installed XP. I have already saved the files I want to keep, so I dont mind the remaining clutter from being swiped clean.