To play on the D2, you are going to have to have Windows XP as a Dualo Boot option on your system.. that way when you start your system, you can choose to start up in Windows XP (not Vista based XP Mode or VM).. but a full blown copy of Window's XP Sp3 as a second boot option.
You can play OP single player, TCP/IP, or on GSA using Vista OS.. but D2 ihas a 2 min lag time every time the map updates as such, breaks playability on the D2 for Vista based users...
By Dual booting your system, you will effectively have 2 HDDs or 1 HDD partitioned in 2 and have XP installed to the 2nd partition.. at boot you can choose OS.. as such, you can choose to boot to XP and install OP in XP and you will be able to play on D2 just fine.. just make sure that you can get XP drivers before doing a Dual Boot on your system and also follow my guide for setting up a Dual Booting machine on Vista / Windows 7 in the Engineering forum located at
http://www.dynaverse.net/forum/index.php/topic,163380252.0.htmlHope this helps.