I don't know if I understood correctly your question, but you are talking about creating a different rule set that will handle Gateway Anti Malware check fr the Windows XP machine, am I right?
Well in my opinion and for my knowledge, there is no difference whatsoever for the Anti Malware to check viruses for Windows XP Machine or windows 7.
The Proxy is an appliance that does not know the operating system of the client machine. It can only know the User-Agent of the browser (from which you can extrapolate the OS).
Said that, for my knowledge, despite your OS, all the malware should be blocked at the Proxy Level and are blocked at the proxy level if the Gateway anti malware rule set is on and properly configured.
Hope this answered your question...
thanks for you posting and please excuse me for answering so late, but I was absent.
What you write is exactly what I wanted to read - thanks a lot. We have some "specialists" in our organisation, who want´s a second opinion, even if all logical arguments are speaking in only one direction.
Thanks a lot!