If I'm interpreting this article correctly, there's no point to enabling hyperthreading on Windows XP Pro or Win2k Pro. Since XP Pro only uses a max of two processors enabling hyperthreading doesn't buy you anything at all, since the OS only uses the first two. Looks like you'd need Advanced Server, which can take four cpu's, to see any real benefit.
http://www.theregus.com/content/4/24276.html
http://www.theregus.com/content/4/24276.html