Hi Dinesh,
Sounds like you may need to do a Repair install. You need a Vista dvd - same bit version as the client has installed.
Repair Install For Vista
If the client has an OEM pre installed version of Vista - some of the proprietary oem apps. may get disturbed - no big loss.
Reactivating may be a slight issue. I assume there is a product key stickered on the client's machine?
You could use that , but might need to ring the OEM to activate.
You could extract the product key and activation code from the HD first, do the Repair install, then reinsert the prod key and act. code. Let me know if you want to find out how to do that.
Are you able to back up any really vital data from the client's machine onto an external HD - before you do anything else?