rehanalisyed

HI there,I'm new to networking and windows 2008. I've created a domain and users in windows 2008. my question is how do i connect a pc either on win xp pro or win 2000 pro from a workgroup to this domain? when i enter domain info in the pc's networking section, it gives me error that domain not found. i can ping the domain server from the other pcs.please help or refer me to any site where i can find this info. google doesn't help much on this.Thanks,Rehan