Indeed, Punkbuster is the cheat detector, and it's not built in, it's a separate process from BF2.exe...
If it is not running or punkbuster cannot be connected to by a server running it (basically all servers) then it'll reject you or kick you immediately/nearly immediately.
And you has BF2 too Kyle? Yayz we can play when I get my new PC (it sucks on this old one).