As long as you use P2P programs, you will continue to have problems.

This thread has links to free antivirus and firewall programs:
http://www.daniweb.com/techtalkforums/thread5690.html
The firewall is nearly as good as Norton without using up as much of your systems resources. Most 3rd party firewalls are better then the XP firewall.

Unfortunately, no antivirus/spy/ad ware program will clean up everything -- you need to have several programs, keep them updated, and scan with them regularly. That thread has additional utilities besides the ones you are already using that can help keep your system clean.

You also seem to have a problem with your memory, as 112MB is not a standard size (and barely enough for XP anyway).