A keychain on Mac os X is basically your mac's way of remembering your passwords, if you don't remember your password(s) then you can use the 2 restore discs that came with your mac, they're gray and they say "restore disc 1, restore disc 2" etc. pop restore disc 1 in and you can reset your password that way, you have to boot from the cd, so if you have to go that route then put the cd in, restart the computer and hold down the letter "c", that will boot from the cd and let you change the password. If you can't remember your password, and you don't have your restore discs, then take it to a mac store cus there's nothing else you can do. It's not a virus btw.