you might be better off upgrading your other PC to W7 as well, and then start to use "homegroup" instead of "workgroup". W7 homegroups are more secure, stable and extremely easy to setup
ps: on a side note, I can't see how a
workgroup name could be remembered after a new setup, that's hardly possible... or see may be
at router level, but I never saw that happen.
(..also, not sure if ZA firewall conflicts with Avast webshield, but this could well be the case)