1. What OS are your DC, include SP level
2. Forest/Domain structure? and Level
3. Is this issue for all Win XP clients and Win 7 or only Win XP?
4. How many AD sites?
5. Run the ADBpa and to check for errors and fix critical.
6. Is this slow logon all the time or random. Pick one client, can you easily reprocue the issue ?
Completely off the wall - it isnt the anti-virus, is it? I had an issue whereby the logons went really slow all of a sudden - slower and slower over the day and it turned out to be the antivirus set to check everything every time the server was accessed. I had recently rebuilt the antivirus and didnt know to stop this.
I'm sure it probably isnt, it is just that your problem is so similar to mine