Not to steal Ray's thunder

, but did you try searching the site. This issue has come up more than once. Go to /home/e-smith/files/server-resources/regedit and copy the winxplogon.reg file, then run it on your xp pc's and then you should be able to join your domain.
In case you don't know how to get the file from your server to your pc, here it is.
[root@test regedit]# more winxplogon.reg
REGEDIT4
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\netlogon\parameters]
"RequireSignOrSeal"=dword:00000000
JB