Anti-Virus Software stops running

Got an odd problem. I’ve had Sophos running for ages on my PC for ages – all of a sudden you would start it up to do a scan and the pgm would just exit, randomly. I removed Sophos and put on Norton Professional 2003 and it does the same ! No other pgms are effected. I’ve disabled the Virus checking in the BIOS and i’ve also run a Scan from the Norton CD and nothing has shown up. A bit of extra info – the PC seems to be getting slower and it wont shutdown properly – just seems to hang on the shutting down screen.