I recently upgraded to Windows XP.

I frequently discover that search misses some files when I'm looking for a phrase inside files. When I re-boot the computer, the missed files show up on the next search.

I have changed my preferences to enable the indexing service and I've told it to index files with unknown extensions.

I'm guessing that the indexing is missing that file for some reason, but picks it up when I re-boot.
What sort of things would cause it to miss a file?
And is there a way to re-index all the files without having to re-boot?

TIA