Windows keeps track of your files for searching. It looks into them and at them to find things it can tag for a search.
It does this at times to update its index. In the past before Vista Windows did this but very badly. It often could find anything or index well. After MS brought Vista they changed the file indexing completely both for speed and accuracy.
Keep in mind that if you have a file but can't find it it might as well be lost. With mega hds out its more important than ever to be able to search them and find a file quickly.
I would go to MS and read up on indexing.