It sounds to me that QlikView is not set as default program for file extension .qvw, for any reason (should have been set during installation, I believe). Have you moved your program folder or have you done a QlikView upgrade in the past (in other words, did you have the default program set right at any time in the past?)?
Anyway, you could set the default program manually, I believe this should fix your problem.
If you are using Windows 7, here is nice description:
If you are using Windows XP or Vista, there are links to appropriate descriptions at the bottom of above referenced web site.
Hope this helps,
Thanks for your response stefan.
It was windows server 2003. It was working fine. It was showing all the QVW files in the Qlikview format. When i closed the QVW file it started showing the QVW files in the unknown format. I just logged of from the server and logged in . It is working fine. I am not sure how it is changed or way to go ahead if face the same issue again.