Controling access to windows XP on a large workgroup

Our company has lots of workgroup. Each workgroup has it’s computer clients. I need a way to control access to all clients in my workgroup only (example: Sales workgroup)so the other computer from other workgroup can’t access my workgroup computer.
My workgroup don’t have domain controller. Only windows XP both Home & Proffessional. Is it possible to do this? any help would be appreciated. Thanks