Whenever I turn on my comp while I'm on xbox live it lags.
Help

Two things I can think of:
1. Turn Xbox live off/logout/whatever it's called.
2. Get better internet connection.
There really isn't anything else to do because a modem can only handle certain amount of traffic without lag.
Well, instead of shutting live you could simply shut all downloads on Xbox, any other devices using internet or any downloads on your computer.

Prorably the internet connection, i reccomend you 1mbps.