While playing World of Warcraft and plant vs Zombie i have noticed a weird occurrence. When i am about to go into a high cpu usage bit of gaming ie. instance running(wow) or a large wave of zombies(PvZ) the computer seem to lock up :o and the avast looks to be scanning the program in use for a minute or so then returns me to regular gameplay 
is there anyway to excluded programs from the shields so i can get uninterrupted gameplay?
Windows Xp
Avast 5.0.667