I first described the subject behavior, and a possible resolution, at tip 5140 » A file that is required to run the Windows XP Search Companion cannot be found?
Another possible cause of the A file that is required to run Search Companion cannot be found error is corruption in the msxml3 library registration information.
To resolve this issue:
1. Open a CMD.EXE prompt.
2. Type the following commands, pressing Enter after each line:
regsvr32 /u /s msxml3.dll
regsvr32 /s msxml3.dll