I'd still invest in your case into Mac protection against passing along malware.
Take a look at Intego AV for one for the Mac and for dual-protection.
http://www.intego.com
Whatever you are finding is 'dormant' I assume, but that doesn't mean it can't become active or passed along either. And there are some 'man in the middle' spyware and malware, some which target online home banking (some people run their home banking from a virtual OS that is used for that purpose only, I always wished Mac OS X could be run in a VM mode as well).
The IT folks sound like they need... do their jobs properly and better.
I don't care how young they are, or the staff's ignorance. Time for education is now.
Download and install Microsoft Security Essentials in Windows and follow guidelines. And there are tons of FAQs, guidelines and articles on the subject.
http://www.microsoft.com/Security_Essentials/
http://www.microsoft.com/security/portal/
Nearly six months after Apple added a malware blocker to Mac OS X (Snow Leopard), the feature appears to be collecting cobwebs.
Apple has not added any anti-malware signature updates to the XProtect.plist file that launched with antidotes for OSX.RSPlug.A and OSX.Iservice, two known Trojan horse programs targeting Mac OS X users.
http://blogs.zdnet.com/security/index.php?cat=20&submit=view