If you do a google search for the "compaq presario SR1602HM motherboard" or something similar, you'll most likely end up on third-party website that sells motherboards for your computer. Most likely they'll also list the part number.
Compaq boards are very expensive, even on ebay. You may have to buy a regular micro atx board and new vista or windows 7 license, which will cost about the same; compaq windows licenses are proprietary, meaning they only work with compaq (hp) boards. I found that out the hard way. Even microsoft won't verify your copy if the windows dvd doesn't match the board brand.