It's hard to tell based on his resolution. Though it does make it seem like he is running an earlier version. But i'm guessing Twisted is not acustomed to seeing the Windows classic taskbar so he assumes it is an earlier version when it could very well be Windows 2000 Pro or Windows XP with a desktop theme set to Windows 98. But there is one dead give away to this theme...
The start button.
http://img108.imageshack.us/img108/8955/winstartph2.pngThis Windows flag design is found on Windows 98, NT, 2000, and ME.
Though, his ability to play Battlefield tells me that this is limited to 98, ME, and 2000 seeing as that NT isn't supported by BF.
The icons in his system tray narrow this field even more. Specifically the yellow one you see at the far right. I believe it is an icon for Norton Antivirus. I believe the version he is running is only supported under ME, 2000, and XP. That brings us to two.
So which is it Masebot, are you running Windows 2000 or ME?