>Aman,
>Check LogMeIn https://secure.logmein.com/
>With it your customer can give you permission for remote desktop assistance.
>He/She can not start the session, but He/She can stop that service and He/She can watch what are you doing during the session.
>Supported window versions: Windows 98, 2000, XP, and Server 2003.

You should check out UltraVNC Single click. It allows the users to call back to you. Free.
http://doc.uvnc.com/addons/singleclick.html