Microsoft stopped supporting Windows XP a while ago, and with the exception of virus signature updates for Security Essentials and the Malicious Software Removal Tool, XP is unprotected as well as unsupported. But a developer has been working on a Service Pack to bring XP up to date with security and other updates. PCWorld has a discussion of it here. It is currently in its third beta version and is available from Softpedia.
This is not in any way a recommendation because I am in no position to evaluate the extensive contents of the Service Pack, and, for sure, YMMV. But I've tried installing it in both an XP Pro VM in Fusion, and, even better, in XPMode in a Windows 7 Pro VM, using the wonder of Snapshots to get me back to their original states while waiting for the "official" release.
Both installations took a while but ran smoothly. And in both instances, the VM's felt more responsive once the installation was finished. XPMode, which had always been sluggish, became actually usable. Indeed, I tried installing an earlier version of QuarkXPress, which is a large program with a multitude of internal files, in XPMode in try-out mode, and it ran surprisingly well (especially considering that it's a nested VM), and almost as quickly as natively on the Mac.
Moreover, the Service Pack opened XP up to updates from Microsoft's Update system beyond the Security Essentials and Malicious Software Removal Tools. And while it's hardly conclusive, after the installation was finished, I did a full scan using Malwarebytes Anti-Malware and then Security Essentials and nothing was found.
There is always the chance that issues or vulnerabilities will be introduced between now and the final version (if they're not already there) and that hacked versions will pop up elsewhere. But publications like PCWorld will be watching and evaluating the progress and legitimate sources PCWorld will likely link to should be safe as long as the Service Pack itself is.
In any case, the performance improvement in XPMode was a real surprise and well worth the time it took to install "SP4."
If anyone else has tried this, I'd be interested in other experiences.