Additional clarification: 2000 Pro doesn't understand the "per socket" licensing, so while a dual core or HT processor (singular) will work fine, anything that puts the number of virtual or real cores over 2 will not be fully utilized. Of course - conveniently for MS's pocketbook - XP does not have this limitation, as correctly stated above.Originally posted by: bearxor
Microsoft has said that they license their OS by socket, not by cores. So if, in theory, you had a quad-core dual-proc P4 machine with hyper threading, XP should be able to recognize up to 16 threads.
So, XP Pro and 2000 Pro recognize two processors. Vista will likely have a Home/Pro edition, which will support 1 or 2 sockets.
Contrary to public opinion, you do not need XP Pro to support a HT processor, or a PentiumD.