XBL gamertag search?

how can i search a person on XBL or windows games live, which has a gamertag i want? :(

on xbox i can write him.. but what if he uses the gamertag on windows games live?

thx bb

p.s. gimme your gamertags on XBL so i can add u :)

@edit: i just tried to msg "beasty" via xbox and got: the guy maybe wont recive your msg cause he hasnt logged in/registered on a xbox or windows games live

WTF?