Question : User is unable to loginto workstations

WHats the FSMO responsible for user logins and what are the stesp to be taken to troubleshoot user login issues

DNS is fine
Communiccation is fin on the n/w

Answer : User is unable to loginto workstations

That is way not enough information to help you.
What is the *exact* error this user is getting when trying to logon?
Can you logon with a domain admin account?
Did the machine join the domain without problems?
Are there errors in the event log of the machine?
What *exactly* do you mean with "DNS is fine"? A ping and an occasional nslookup doesn't determin "fine". Please post the result of an "ipconfig /all" from that machine.
Some preliminary information about DNS, since most logon problems are, after all, a result of incorrect DNS settings (do *NOT* use external DNS servers in the TCP/IP configuration on *ANY* domain member!):

10 DNS Errors That Will Kill Your Network
http://redmondmag.com/features/article.asp?EditorialsID=413

Frequently asked questions about Windows 2000 DNS and Windows Server 2003 DNS
http://support.microsoft.com/?kbid=291382

Best practices for DNS client settings in Windows 2000 Server and in Windows Server 2003
http://support.microsoft.com/?kbid=825036

You can use netdiag.exe and dcdiag.exe from the Support Tools for further troubleshooting:
Windows Server 2003 Service Pack 2 32-bit Support Tools
http://www.microsoft.com/downloads/details.aspx?familyid=96A35011-FD83-419D-939B-9A772EA2DF90&displaylang=en