ok, so you already have a workgroup?
Then just right click it (properties-> sharing and security) and check share.
Then open windows explorer -> click tools -> map network drive -> browse and look for it.
I suggest you give it a letter at the end of the alphabet cause if you connect and external drive it is going to call it D(if you only have a C drive) and if there is a network map that is already called that way you will have troubles.
Jonas