Ive had two machines recently which would go through the boot process until the windows logo appeared then reboot.
I entered the covery console on both and ran a dir on the system drive but it threw up an error along the lines of 'unable to enumerate'. I proceeded to run a chkdsk /r on both machines which resolved the problem.

Out of curiosity i looked at the application logs in event viewer just prior to the chkdsk and found 4 errors from source msiinstaller.

Error1
Code:
Product: Microsoft Office Professional Edition 2003 - Update 'Update for Outlook 2003: Junk E-mail Filter (KB931764): OUTLFLTR' could not be installed. Error code 1603. Windows Installer can create logs to help troubleshoot issues with installing software packages. Use the following link for instructions on turning on logging support: http://go.microsoft.com/fwlink/?LinkId=23127

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Error2
Code:
Product: Microsoft Office Professional Edition 2003 -- Error 25090. Office Setup encountered a problem with the Office Source Engine, system error: -2147023838.  Please open C:\Program Files\Microsoft Office\OFFICE11\1033\SETUP.CHM and look for "Office Source Engine" for information on how to resolve this problem.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Error3
Code:
Product: Microsoft Office Professional Edition 2003 - Update 'Update for Office 2003 (KB907417): OTKLOADR' could not be installed. Error code 1603. Windows Installer can create logs to help troubleshoot issues with installing software packages. Use the following link for instructions on turning on logging support: http://go.microsoft.com/fwlink/?LinkId=23127

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Error4
Code:
Product: Microsoft Office Professional Edition 2003 -- Error 25090. Office Setup encountered a problem with the Office Source Engine, system error: -2147023838.  Please open C:\Program Files\Microsoft Office\OFFICE11\1033\SETUP.CHM and look for "Office Source Engine" for information on how to resolve this problem.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Unfortunately i didnt check the event viewer on the other machine but its a bit of a coincidence it happening twice in the one week!

Any thoughts?