99% sure i have a virus

ok, i do not know much about computers or viruses but after reading some of these posts i hope someone can help.
I have a computer running XP that uses norton antivirus. at this point this is what i need help with. i can boot up the computer but as soon as windows starts the norton scheduled scan begins and the system automatically restarts. I tried booting in safe mode, safe mode with networking, last known good, and all these stall, booting normally will start the system to the point that the AV scan starts and then I am back to square one. I know I have a much bigger problem here, but I don't know how to even begin if i can't boot up the system. Also, have maybe spent 10 minutes of my life in MSDOS so if that is how i have to go I need very specific instructions. Thanks in advance for anyone that can help me out here.

oh one other thing, if the infection is really really bad, please just tell me if i should toss it hate to waste my time on a lost cause:P