I am not able to "run as different user" in my windows 7 (domain login) machine.

For example if i'm trying to start iexplore (run as diff user) It returns the error :

"The system cannot find the file specified".

Please help me. thanks in advance.

Does the other user have the appropriate permissions to do so? Does your account have sufficient privilege to change other user's settings?

Check if you are running as a standard user or as an administrator in the Control Panel. Standard user can't run other user's programs.