Choosing Domains

Hi,
I hope I explain this correctly.
I have a windows xp,pro. Laptop.I have a network domain that I connect to. I just went to another site and connected to there domain. So I set up my computer with the new domain name and connected.
Now whenI go back to my 1st domain(which I have not done yet) I guess I have to rename the domain
to connect. My question is do I have to keep doing this each time I go back and forth between sites, or is there a way to have maybe a drop down box giving mea choice of which domain to choose.. If so how would I do this?
Any help would be appreciated.