Active Directory login takes from seconds to minutes on OSX
There are several ways to get a Mac to work on an Active Directory network. Apple has some issues with it's Samba implementation that seem to cause these sorts of issues. I have been using software from Likewise to join our Macs to our domain. Starting with version 6.1 (currently in pre-release) the issues that you describe are greatly reduced (not eliminated) -- it still takes me about 45 seconds to log in.

If you have administrative control over your Mac, then you will certainly want to read up on migrating from your current AD implementation to Likewise. If you are not a admin, then you will need to speak to your IT department. It is important to note that Likewise should be installed as a local admin, and upgrades should not be applied as a domain user (even if they are an admin), but as a local user.