Norton Virus Using Definitions from A:

Hi,
I want to remove a virus from a pc. I inseert the NAV 2003 cd in the cd rom and boot from the cd. When it boots I run the anti virus scan. On the mopnitor it says that it is using the definitions from the A: drive yet there is no disk in the A:drive. Am I suppose to download definitions from Symantec onto a floppy for this procedure to work. I ran it off the cd with nothing in the floppy and it says no virus found. Ther pc does not let me install NAV and run so I suspect it is infected.
Thanks