I am using Compaq desktop having XP as OS.. I am not having TV Tuner driver for it. On compaq's official site for driver it asks for product no. From where can I get the product no.?

have a look at this link Compaq TV Tuner driver it might help

Step-by-step guide for install Compaq TV Tuner driver.
1. Download file for Compaq TV Tuner from our site, save it on your computer and unzip.
2. Connect your Compaq TV Tuner to computer.
3. Right click on My computer and click Properties. Go to Hardware tab. Click to Device Manager button.
4. Find your new Compaq TV Tuner on nodriver category double click on device and press Reinstall Driver button.
5. Select Install from specific location, check Include this location and click Browse button to select the Microsoftdriver click Next button to continue and click Finish button to finish installation.
6. After this procedure you should see new Compaq TV Tuner in nodriver inside device manager.

Get the step by step process and the product number from the site

Compaq TV Tuner driver

tv tuner - Software Downloads - TechRepublic