Put this as your screensaver and use ShapeShifter or ThemeChanger with a Windows-esque theme such as Mac OS XP 2.5 (http://macupdate.com/info.php/id/13995), hide the Dock, cover your Mac logo somehow (I have my G4 tower under my desk anyway), use Office 2004 with Entourage 2004, and you might be able to convince people you're using a PC.
No, wait, you'd have to pretend to have your computer crash way more often, fake the occasional BSOD, and curse at it...
(Warning: the Mac OS XP 2.5 theme may not play nice with Tiger (see the comments for it; hopefully a 2.6 or 3.0 version will come out), but we still use Panther where I work so we can use ADmitMac 2.1 on our Windows Server 2003/AD network) as ADmitMac 3.0 for Tiger is still in beta.)