This applies to Windows Vista, Windows 7 and Windows 2008 servers.
This usually happens after a windows update as was for me but also occurs when you update some drivers. Some commands which requires a reboot.
Your OS might scare you when you reboot and it says: "Configuring Updates: Stage 3 of 3"
Dont worry try the following :-
1. Get into advanced boot options by hitting on F8 key. Try "Last Known Good Configuration". I promise this will not resolve your issue, but still it's worth giving a try.
2. Now that doesn't work. Get the respective OS installation CD and insert it into the CD drive. Reboot the computer and again get into advanced boot options, and select "Repair your computer" choose "Command Prompt" in the recovery choices.
3. Enter the following command:-
del C:\Windows\winsxs\pending.xml
4. Exit and reboot.
Good luck !!!!