As FunkyPengion mentioned, postfix doesn't (and won't, as far as I understand it) exist for Windows and VMware is your only hope of running a linux & Zimbra on XP, You'd need a fairly large amount of RAM (real and virtual) for Zimbra to run well. If it's only for a couple of users then you might get away with a smaller amount of RAM.
VMware server is actually free these days, have a look in the forums for a post from me with a link to the VMware website.
If you want to try Zimbra without installing linux in VMware have a look at this
page and get the Virtual Application. Zimbra is already set-up and you'll just need to install and use VMware to run the appliance.