A problem with group policy can cause Windows to prevent a user from logging in to the system. This issue is documented under KB 941158 as a fix for a 20 second login delay issue. The hotfix is included in XP SP3, Vista SP1 and Server 2008 SP1, however, it's not turned on by default according to our friends at MS PSS.
You'll need to boot in safe mode and tweak the registry in order to have the fix take hold. Take a look at the MS' knowledge base article for specific details.