Defwatch scan taking a long time

When a new virus definition file arrives, defwatch takes about 40 minutes to scan, and slows everything down to a crawl in the process.

Active Scans run through the UI only takes 2 minutes.

This problem seemed to start when I installed multiple bootable partitions on the computer. I normally boot a XP partition, but there is another XP partition, a Win 7 partition and a data partition (total of 4).

Any ideas what could be the cause?