|For starters, 240MB isn't enough RAM to run XP decently, so you should expect the system to run slowly. But obviously 100% CPU usage & taking 15 min to post a question indicates that you have other issues.|
Considering how little RAM you have, you should be running the system as lean as possible. That means you shouldn't have a lot of crap loading at startup & running in the background. In other words, no instant messengers, no Quicktime, no iTunes, no RealPlayer, no camera/scanner/printer software, no toolbars, etc, etc. You should be using a lightweight anti-virus program & MalwareBytes should only be used when needed, it should not pre-load.
Did you check to see which process is hogging the CPU? When you ran the scans, did you do it from safe mode?