I am looking for a VoIP provider which 

* provides the ability to use software running on Microsoft Windows PCs to receive
and make phone calls

* has a monthly unlimited USA calling plan

* provides service to the 205 area code. Thanks.

I don't know of a specific provider with all of your requirements, but
you might want to check this site.

http://voipreview.org/

It allows you to compare services from multiple providers and set local area alerts.

Vonage does allow you to use software, called softphone on your laptop
or desktop in order to make and receive calls via your PC. I've never
used it but I have friends who have and they rave about it.

Vonage has monthly unlimited for 24.99

Vonage will provide you service in a 205 area code, however, the only
numbers they can give you are 215 numbers, which I understand is a
mobile prefix for all of Alabama.

If you'd like to join vonage, message me with your email address and
I'll send you an invite which entitles you to one free month of
service on Vonage. Thanks!

I've found that this site provides good information on the Vonage
product: http://www.vonagevoiceplans.com

It also allows you to ask questions regarding the Vonage product and
get a response via email.