Ganesh,
First thing you will want to review is the server's event logs to see if there is anything obvious that may be causing the slowness.
You should also review your Windows Update history to see if there were any updates installed recently that may be causing conflicts.
Past that, you will need to use some tools to trace and analyze your servers usage and performance.
A few questions:
Are all sites on the server slow?
Is it just websites that are slow or is the entire server bogged down?
Are there any specific types of pages/files that are slow? (eg. .NET, PHP, HTML, images)
Have any changes been made to either the server configuration or coding in the last week?
I would recommend the following tools to help track down the performance issue:
Server Performance Advisor
http://www.microsoft.com/downloads/details.aspx?familyid=09115420-8c9d-46b9-a9a5-9bffcd237da2&displaylang=en
IIS Diagnostics Toolkit (x86) -- (specifically Debug Diagnostics)
http://www.microsoft.com/DOWNLOADS/details.aspx?familyid=9BFA49BC-376B-4A54-95AA-73C9156706E7&displaylang=en
How to use Microsoft's IIS Diagnostics Toolkit
http://searchwindowsserver.techtarget.com/generic/0,295582,sid68_gci1276937,00.html#
Hope this helps!
Brock Hensley
Technical Support Representative
http://www.ServerIntellect.comManaged Servers, 24x7 U.S. Support, Web Hosting Solutions.