We are using windows xp in our company with 40 computers networked in a single lan,
We have created workgroups like sales, accounts etc.. will u please give guidelines to setup
Password for r workgroup so that only people in the same workgroup can access the files.
That is only sales people can access computer in the sales workgroup and not accounts workgroup.

You would be much better off using a domain then workgroups. Workgroups are not really meant to be used with so many computers. A domain would allow for much more control and security.