Make sure that your AV Definitions are up to date. There is nothing on the Symantec or Mcaffee sites about this problem. Currently there are no Virus's for OS X
Try booting from the Norton CD and running a scan from there. About the only thing that might mess with your machine is Hacktool.RootKit but it is a very rare occurrence.
You could also try starting in "Safe Mode", Hold down the Shift key after the chimes and see if the problem continues to happen.
Worst case scenario, Reformat the drive and reinstall the OS. You can always restore your data from your backups, right?
I have a Powerbook G4 Titanium and as of yesterday, it has been acting strangely. Every time I start it and open ANY program, it runs (somewhat hesitantly and slowly) for a short while (a few minutes) but eventually it freezes and I get a message that I need to restart the computer. Running the Disc Doctor repeatedly and fixing damaged files did not help. I managed to run Norton antivirus once and it did not detect anything. Other attempts to run the anti-virus all ended in the computer freezing and telling me to restart. I tried to open Safari and Explorer but both efforts ended with a virus alert and subsequent freeze -- apparently it is hacktool.underhand and the computer cannot fix it or delete it. Is it possible? Can anyone help?