Cant connect SQL server by Enterprise manager but no problem by Query analyzer

I cant connect SQL server by Enterprise manager, then i try to connect it by Query analyzer it works !
I have 2 SQL servers get the same problem, no matter how many time i try,
Even I set the timeout to900 seconds, I still got the message in Enterprise manager..

Help me please..!

A connection could not be established to xxx.xx.xxx.x

Reason: Timeout expired.

Please verify SQL Server is running and check your SQL Server registration properties
(by right-clicking on the xxx.xx.xxx.x node) and try again.I have been able to get this working by going to the Client Network Utility and switching the primary network library from TCP/IP to Named Pipes or vice-versa. No idea why some machines prefer one over the other, but life would be boring, if we did not have these little things to keep us up all night ;-).
~Matt