BSNL - Broadband Account Management
That will let you change your password. You are on the new National Internet Backbone II of BSNL. You must have got a 14 digit Portal User ID from BSNL. Use that with your password to login. Then you can change your password.
DHCP is failing due to your WiFi Cards drivers. Try updating them. If problem still persists, see if you have some utility from your laptop or WiFi card's manufacturer running. Disable it. Then use Window's Zero Configuration Utility to enable DHCP and try.
If you have only 1 or 2 systems, I would suggest turning off DHCP on the modem and using static IP addresses on both, desktop and laptop. Makes things easier to troubleshoot.
Congrats on getting things to work.