I normally don't post technical articles, but I found very little info on the internet relating to this problem.

Problem: Windows XP professional will join a domain running samba, but once rebooted, will not log into the domain.
NOTE: This same scenario will occur when joining a Windows NT server domain running service pack 3 or below.

Windows Error Message: Windows cannot connect to the domain either because the domain controller is down or otherwise unavailable or because your computer account was not found.

Samba Error Message:
[2002/04/22 11:46:21, 0] rpc_server/srv_netlog_nt.c:get_md4pw(176)
get_md4pw: Workstation greg$: no account in domain

Cause: Windows XP-based clients try to sign or seal the secure channel by default, but Samba does not support this.

Solution: This is straight from Microsoft's Knowledgebase article (Q318266):
Click Start, and then click Control Panel.
If you are using Classic view in Control Panel, double-click Administrative Tools , and then double-click Local Security Policy.
If you are using Category view in Control Panel, click Performance and Maintenance , click Administrative Tools , and then double-click Local Security Policy.
Under the Local Policies\Security Options node, double-click the Domain Member:Digitally encrypt or sign secure channel data (always) policy to open it.
Click Disabled , and then click OK.