Its probably just the way the new viruses are written. Once they are active, its hard to terminate them. I suggest booting into safe mode to clean any infected system. Scans are usually faster, and you know the system is clean when its done. If you are running XP, don't forget to disable system restore first.
Matt J.
Please always take the time to backup any and all data before performing any actions suggested for ANY problem, regardless of how minor a change it might seem. Also test the backup to make sure it is intact.