Boot Record keeps changing

My antivirus software, Norton Antivirus 2003 Pro, keeps informing me when I scan the computer that the Boot record has changed. I always select that the change was unexpected and choose to restore. It scans and finds no viruses, yet this still keepshappening. I don’t believe I have any software that would cause this. I have a slave hard drive. Also I might note that when this system was built, it recieved a virus before I could install Norton, the virus caused Norton Install to fail, and I believe is causing the Rescue Disk utility not to install. Would my best bet be to reformat and start anew, or is there some kind of advanced virus software that could save me? Thanks.