Some PCs on Network are Freezing

We have a network with a domain and approximately 30 machines (primarily XP Pro, some Win 7 Pro).  There is a Win  Server 2008 R2 domain controller/ file server.  Starting on Monday we had two machines that worked for a few minutes after login and would freeze (mouse can move but clicking on anything doesn't work).  

I was able to do a virus scan (nothing found) and then system restore on one machine and it is now fine.  System restore did not have an effect on the other machine.  Attempted to repair the windows installation on this machine but it just hung after putting in the admin password.  I even changed the admin password without luck.  This PC works ok in safe mode.  Two additional machines started having the freezing issue today.  One would not go into safe mode at all and the other did a couple of times.  After that the second PC would no longer go into safe mode.  A blue windows boot manager screen would come up and ask you to choose the windows installation to boot from.  It will take some time to boot up and you will be able to login and then have a minute or less window before it freezes.  I also restarted the DC to help rule out a down service/login/profile/time issue.  It made no difference.  All other machines on the network appear unaffected.  My best guess now is a bad Windows update.  

Any ideas?

Thank You