You can only play if:
- the modchip is disabled of course, so therefore you have to use an original copy of a game
But, have you been on Xbox Live before at all? Do you have a new HDD installed now that you have a modchip?
If this is the first time you're considering going on Live, and the first time you're gonna access the Live in the xbox dashboard to register and all that, then you must remember that modchip must be turned off, and that the HDD you use to register (or even just access xbox live trying to register) is the one you'll have to use to play on Live. If you've used a different HDD in the past to play on Live, you have to use that HDD.