i’ve heard that av resident scanners can slow computers down,i use a p4 512mb ram 160gb hdd with 32 mb onboard graphics and i am used to get a little slow response from the computer when it tries to initialize avast when it is switched on and also when i play GTA san andreas ,do i have a problem here?
your obious shortfall is in graphics from my obsevation . Most systems of those standards would have a graphics card with around 256 mb so your`e way short of the mark. It wont change the speed of Avast much but it will certainly help your PC render pages a lot faster/ better, especially with games.
good luck 
As Cloussau said your limitation for games is your on-board video graphics, there are people paying £200 and more for the latest graphics cards to play the latest games, so when you talking about a motherboard that costs in the region of £50 then you aren’t going to get much in the way of graphics power.
However, when you boot your system, avast1 scans files as they are called before they open and this will slow the boot a little, you should also notice that the avast icon when it is displayed is rotating indicating it is scanning files. You would be best to ensure only those programs that are essential are allowed to start on boot.
You don’t mention the sensitivity you have set for Standard Shield, Normal is the default providing a balance between performance and protection. The higher that is set the more files will be scanned.