If you bought your computer and it came from a reputable manufacturer (i.e. your computer has a legal version of windows) you should be able to find a sticker somewhere on the computer that has your product key on it. The sticker will be blueish green and should have some indication it's from microsoft.
If it's not on the computer itself, it may be in the documentation for the computer, or on the CD case for windows. If you still can't find it, contact your PC manufacturer and they should be able to help you find it (or possibly have the information on record for you).
If you pirated Win7, or bought the PC from someone who may have pirated it, you are out of luck. If this is the case, buy a copy of windows for yourself.