Actually, if MSFT did what they claim to, XP will "appear" to run logon scripts slower than W2K machines... They say that, in order to boost boot up time, you can get to a "press ctl-alt-del prompt" BEFORE the network is fully initialized. Problem is, when you "need" the network in order to run scripts and perform AD lookups or such, you'll have to wait for the network components of XP to be fully operational.
Don't know if this may help, but you could try setting up a W2K Pro machine, run some tests on it, then upgrade it to WXP and run the same tests.
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Later,   [b]Mad[/b]ruga