Yes more ram would make it run better. But on a clean instal of xp will run quick even as low as 128mb of ram but alot of hard drive churning though!.
Must be alot of start up items try msconfig and reduce the group start up items to try and gain some speed?
How much free space is on the hard drive? if it is less than 18% free you will start to have problems running windows. System restore points take up alot of room on the hard drive. try cleaning out them?/ clean out c:/windows software distrobution /download files folder (will contain GBs of junk in here (safe to remove)
ah avast did find something and moved to chest and must be something on the computer then!