Norton for Mac frozen

Hello,

I recently installed the Norton for Mac antivirus software on an iMac 9.1 with OS 10.5.6.  After the first scan it detected several viruses and I hit "repair all".  Most viruses were repaired but 4 were shown for "repair failed".  Each of these 4 were in a "cache" folder.  These are all some sort of Trojan virus (as listed).  After the next scan only these 4 viruses showed up again and again the "repair failed".  The third scan I decided to delete the files.  When I chose to delete the file, Norton asked me to type in my system admin password and following this it showed that the files were deleted.  Now I cannot quit Norton, or shut down the computer.  Everytime I go to turn off the computer it says I must first quit Norton, which I cannot.  Even hitting the power button does not work, as when I turn it back on, the computer comes back to the same screen and does not go through the start up process.  Any suggestions as to how to fix this mistake of deleting the viruses?

Many thanks,

Brandon