If given the choice ... XP Pro. Home lacks the ability of Group Policies, which while you don't have to be IN a group, can have an effect on the pc as a whole (easily set the policy to never allow MS Messanger to be run, to easily set additional security logging parameters, and to easily disable LMHash, Enable Admin Login under recovery mode, and enable admin full access to drives/folders in recovery mode) ... oh and just for the simple fact that you *can* install the recovery console ... any time someone gets infected ... its funny how you can go into recovery console, and nix a few files that wouldn't otherwise be running except they get started on normal boots. But not so under Vista or 7 ... you have to have the "recovery boot disk"

much of this can be done with boot disks, and registry hacks.. but the Group Policy Editor makes it so much easier..

Next on the list would be Windows 7 pro ... The thing vista got wrong, and continues to have at its core, is UAC. Its such a hassle on Vista, that its likely to be turned off... under windows 7 however, its less likely to annoy you ... and shoudl someone else come up to your pc for a few minutes, if you already have UAC off, they won't know and they'd blame you for turning it off, as to the infection they got on you when they were just checking their email..