3 identical machines get stuck during log on

We have 3 new Dell gx520 machines for the staff and this morning they have stopped logging on properly. They get stuck on applying personal settings and get no further. I thought it was a network issue so I have moved onto another segment and still the same. All other machines are behaving so I dont think it's a DNS issue or anything like that.
Theres nothing that sticks out in the even logs either ...

Any ideas?

XP sp2 fully upto date afaik

I've had this. Turns out the profile keys were corrupt in the registry and I had to reimage the machines in the end.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileGuid

Make sure all the subkeys are browseable and there are no duplicate keys. If anythings wrong you'll get access errors. Reimaging seems to be the only solution.

:x :x :x :x :x I thought id try uninstallling sophos V5 and guess what.... Logged on fine after the reboot. Oh well.
Thanks for the post though will be something to look out for in the future.