gagan garg,
Do you use windows 7 Beta (Build 7000).
If this is the case this is a normal behavior, this is because windows 7 beta is expired.
You can find more information about this behavior on the following link.
http://windowsteamblog.com/blogs/windows7/archive/2009/05/26/clarification-on-the-date-for-bi-hourly-shutdowns-for-the-windows-7-beta.aspxDoes this solves your problem?
Kind Regards
DFT
IM me - TWiTTer: @DFTER