I could see where some folks might want to still use an old rig with XP installed for the software that wouldn't be compatible with newer platforms. But like you mentioned anyone foolish enough to do ANY on-line activity with XP is, literally, taking a huge gamble even if you have an AV installed or not!
Of course Immunet quit supporting XP years ago. This includes newer & older builds of Immunet including your version 5. Because of this Immunet will not even be able to reliably connect to the Immunet servers for cloud look-ups during scans or pull down new ClamAV malware definitions. The servers won't recognize the in-coming requests as legitimate.
I'm sorry to say you had to find this fact out the hard way Steven. You should have asked first on the forum 'before' spending all that time experimenting!
Here are some AV solutions that might still work with XP & still have some decent efficacy according to a recent AV comparatives test.
BitDefender Internet Security 2014, Kaspersky Lab Internet Security 2014, and Panda Security Cloud Antivirus Free 3.0.
If you can find an older installer of Malwarebytes Anti-Exploit that supports XP that would be a great added layer of security for users.
I helped closed alpha & public beta test Anti-Exploit for years when it was a Zero Vulnerability Labs product and then for Malwarebytes after they bought the company. For my efforts I got a free lifetime license for Anti-Exploit Premium (the paid version)! Is that cool or what!
A-E is now part of MB's AV but you can still get new public beta builds to test.