Yep FAT32 doesn't support user write permissions, sorry to hear you're stuck with it, it belongs to a past era, which is fortunate for security.
Anyway, messaging itself doesn't entail so many risks that you can't control if you're sensible. The only protection AV programs grant to IM is scanning the received files. I don't know if you transmit files via IM a lot, but needless to say you should accept files only from people you know and trust. Accepting a file from an unknown source, even if it's scanned with an AV (which can always miss one nastie), automatically or manually, is playing Russian roulette, not to mention that an unknown party's offering you a file is quite a suspicious behaviour; you should never accept no matter what protection you (think you) have.
Other risk which IM entails is, just like accepting malicious files, clicking on malicious links. Here it's all up to your browser and whatever protection you have (AV etc.) for your browser, not the IM program. So as far as I know here it doesn't matter what IM protection your AV grants you.
If the only thing you're doing via IM is transmitting text messages as is the main purpose of IM clients, then you don't have to worry much: text cannot perform attacks. As long as you stick to bare messaging, IM is quite safe.
Finally, there's another risk, as with any program that accesses the Web, and that's whatever code vulnerabilities your IM cilent has that can be exploited. Again no AV can protect you here, since all they do is scan files and the attack here would include no files, just code injected directly into memory. The only useful things would be, of course having few or no vulnerabilities in your IM client, and some protection like Comodo Memory Firewall that would stop most of these attacks that your AV can't protect you against.
(So in answer to your question, Comodo Memory Firewall can secure instant messaging.)
However as far as I know no attack is possible with plain text messaging. The only buffer overflow attack I heard about in IM needed the victim to accept a webcam session invitation. Again, text cannot perform attacks. Like when accepting file, accept invitations to use advanced non-text features (webcam, blackboard etc.) only from known and trusted sources.
Author