I am using my netbook with BSNL Wi-Fi Modem. the modem has 4-5 ethernet ports on the back but no USB port. Is a changeover cable/ adapter from ethernet to USB possible as my desktop does'nt have an ethernet port.

Hi, using a Network card is best solution in long run. If you have high speed broadband connection, you might not be able to use it at full with USB connector.
Anyways, there are USB to ethernet adapter available around at cost of Rs. 100-200 only. You can use it. You can find here : HERE

Also, a normal 10/100 mbps LAN PCI card cost similar in price.