I recently repaired a Windows XP installation for a friend. The machine would not boot into Windows and was complaining about a missing system files. Running the Windows XP repair tool resolved the problem, however LCK and EXE files would still not work correctly.
The solution can be found at the following URL:
http://www.dougknox.com/xp/file_assoc.htm
This site also has registry fixes for a bunch of system file associations. In my case running the EXE and LCK repairs worked prefect.
