Virus takes scan 8 hours and increasing weekly (only 20% disk space in use)

How long should my weekly full virus scan take?

I have Norton and am running Windows XP Service Pack 2, IE7. I only have 50GB of 227GB disk space occupied. However, that apparently contains over 1,200,200 files. The full scan is now taking some 8 hours and increases every week a little more. The quick scan still only takes about one and a half minutes, that has never increased. I'm not supposed to have any viruses at the moment.

I use Norton version 12.8.0.4 for Windows XP BT Yahoo Broadband