While there isn't much to go on here, I'd tend to fall back to my default troubleshooting mantra..."95% of all problems are Name Resolution until proven otherwise!"

So my first thought would be, "Where are the XP Clients pointed to for DNS?   Is it the same servers as the Domain Controllers?   Do those DNS Servers host the zone for the domain?   Did the DCs properly register their records in DNS?"   On the DCs, try running "dcdiag /v" and looking for errors.    Also, run "dcdiag /test:dns /v" and verify that it is reporting that all of the records are registered properly in DNS.   If they are not, then that is the first thing to resolve.

After you get DNS straightened out, make sure you've got proper NetBIOS resolution as well.

John

That was also the first thing that turned up in my mind... My domain controller is also my dns server. Dcdiag does not show any errors in DNS. All my event logs are error free.

The funny thing about this is that my vista clients use the same dns server and do not experience any dificulties. I will try adding a windows 2003 dns server to the domain... just to see if this solves my problem...

What do you see under the _msdcs.<domainname> zone in DNS?   If the problem is a lack of records in DNS (particularly the SRV records under _msdcs), then a W2K3 vs W2K8 DNS server should make no difference.

What about running Netdiag & DCDiag from the XP Client?   Any errors reported there? 

 One of the reasons why Microsoft synchronizes the releases of Windows Vista SP1 and Windows Server 2008 is the need to emphasize a high level of interoperability between the latest Windows client and server platforms. A level which is not reflected in the interaction between Windows Server 2008 and down-level operating systems such as Windows XP and Windows Server 2008. And in order to make all Windows products play well together, Microsoft has released the Windows Server 2008 read-only domain controller compatibility pack in mixed environments running XP and Windows Server 2003.

"To provide support for mixed mode operations (Win2003 & Win2008) in domains involving

Win2008 DCs, all the features are made available for downlevel clients (XP and Win2003) also. However, since XP and Win2003 were not developed to support the additional checks and flags introduced in RODC, some of the features fail to work with downlevel clients are interacting with RODC. This is more apparent in Demilitarized (DMZ) networked branch offices. In this kind of setup, the clients are restricted access ONLY to RODCs and not other write-able DCs, outside the network," explained Jane Lewis, Platforms Engineer Customer Environment in the Premier Field Engineering at Microsoft.

Essentially, what Lewis pointed out is that there are inherent compatibility problems between Windows Server 2008 and older versions of Windows clients. This is because neither Windows XP, nor Windows Server 2008 feature support for Windows Server 2008 RODC features. According to the Redmond company, the issue affects XP SP2, XP SP3 and the 64-bit version of XP, as well as Windows Server 2003 SP1 and SP2. A total of five updates are available for download, designed to integrate with all the impacted operating systems.
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