How to remove workgroups?

I just got a new Vista machine, running Home premium. I'm trying to network
it to 2 other XP machines (already networked with ethernet through router).
During this process, to organize and I hoped simplify everything, I renamed
computers and workgroups. I now have everything in a workgroup called
"workgroup" and computers are now new names that work well for me. However,
it didn't just change the names, it created new ones. So I now have 3
computers with new names in a new workgroup, and 3 computers an old
workgroup, even though they can't be accessed. How do I eliminate the old
workgroup and computer names? I can't find a solution on any XP or Vista help
page. The phantom names show up on all three computers now. I get error
messages saying they can't be found when I click them. Thanks for any help.