How to solve dwm-156 dial up failed

I am sure it is now toast and you cannot downgrade if the wrong firmware has been installed.
I would ask D-Link.

http://support.microsoft.com/kb/122839

check, there is solution

The USB Adapter is plug and play. This means Windows will automatically detect it and start installing the drivers for it. If Windows is unable to install the drivers you might have to install the ones from the CD if one was provided along with the USB Adapter.

Please note that according to the specifications I found the D-LINK 3.75G HSUPA is only compatible with USB 2.0. This means that if you plug it into a USB 3.0 port it won't be detected by your computer and the USB Adapter will not work.

It could be that the SIM is not functioning. Try it in a phone. If it works then the DWM is at fault, else ask Airtel to replace the SIM.

install modem driver from motherboard cd

ple solve error id 734

Error 633 D-Link (DWM-156) modem

-my d link usb modem dwm -156 3.75g cannot detect the computer

I would suggest that you go through this guide, it will explain you every step which you need to make to get your modem and internet connection back on track:
The ultimate Link Troubleshoot guide: Problems. solutions, PDF manuals

restore your system or repair/change your os... if still doesn't work den change your dwm 156