OK, you've configured your clients as both Firewall and Web Proxy clients, the ISA Firewall is a domain member, and the users have correctly logged into the domain. Everything looks good but you're still ask by the ISA Firewall to authenticate! What's up with that?
It should be that incorrect client credentials are being cached on the client computer. If so, do the following:
Solution: Clear the cached credentials, as follows:
1. Click Start, and then click Run.
2. In the Run dialog box, type control keymgr.dll. Then click OK.
3. In the Stored User Names and Passwords dialog box, select the entry that you want to remove, and then click Remove.
4. Click Close to close the Stored User Names and Passwords dialog box.
5. Restart the client computer