That would be running Spybot-SD to max or near max priority. In my perspective that is not good for the processor as it only has 256MB of RAM. The average Windows XP OS (as I know it) uses a 512MB as minimum. The maximum is 3GB for Windows. 4GB for Vista. To the point...
http://forums.spybot.info/showthread.php?t=27502&page=2
This link solved a member's slow scanning problem.
Please try that.