Microsoft has released a hotfix for a recent problem with long domain names that was discovered in Windows 7 Beta. It is apparently not possible to connect to a domain in a network that is using a domain name with more than 15 characters. The problem is not only affecting Windows 7 but also Windows Server 2008 R2. If you are experiencing this problem in Windows 7 you are asked to download and install a hotfix that solves it.
Below are direct downloads that will fix the problem on 64-bit and 32-bit Windows 7 systems.