have a xbox 360 pro with 60GB hard drive 1 month old can't play original xbox can any one explain why?

If you sign in to Xbox Live, you should be able to download an update to play certain Xbox original games, assuming they're on the list found here:
http://www.xbox.com/en-US/games/backwardcompatibilitygameslist.htm
Or just check out this page:
http://www.xbox.com/en-US/games/backwardscompatibility.htm