2GB of RAM on XP is plenty, that is all I have on mine.
Personally I think a Quick scan would be better suited to your needs.
- With a resident on-access antivirus like avast, the need for frequent on-demand scans is much depreciated. For the most part the on-demand scan is going to be scanning files that would be otherwise be dormant or inert. If they were active files then the on-access file system shield would be scanning them before being created, modified, opened or executed.
I have avast set to do a scheduled weekly Quick scan, set at a time and day that I know the computer will be on. If for some reason my system wasn't on, no big deal I will catch up on the next scheduled scan.
That said it doesn't hurt to do a Full scan when you first install avast, but don't change any settings like scanning archives or scanning all files or it will take a very long time.