Aha.... Gotcha...
So Anthony is correct. You need to have your Mac "bound" to the AD domain. Really not a big deal to do... its rpetty easy in fact...
1. Make sure that you have all of your computers' time sync'ed. I have one of my domain controllers setup as an SNTP server. I define that computer in our Mac's date and time applet as the time source.
2. Go to Utilities/Directory Access
3. Click on Active Directory and click Configure
4. Setup your specific information. (Really just computer name and domain name are required for most AD installs)
5. Click the Advanced Settings button, and review the options. Set them to your liking.
6. If you have any AD groups that you would like to have administer the computer, set them up - you should see Enterprise Admins and Domain Admins by default.
7. Click Bind. This will add a computer account to AD for your Mac.
8. You will be prompted to enter an adminitrator's username and password to complete the operation - this is a AD admin it is asking for.
This should do it... you can now log into your Mac using a Windows user account. You will then be able to access network resources (fileshares, printers, etc) without having to enter your username and password again. (that is - until your kerberos ticket expires - but hopefully you log off at night...)
Here is a doc on Apple's site about it... very simply stated...
http://docs.info.apple.com/article.html?path=DirectoryAccess/1.8/en/c7od45.html
HTH...
David