For future reference - FenderGtrsRock is on the right track. It's not a bug, though, it's a "feature." An annoying, useless feature in my opinion.
When you open a folder through My Computer or Windows Explorer, XP starts indexing everything in that folder. While this is going on, the files are "in use."
In the case of big files...as .avi files tend to be...that indexing can take quite a while. If there are a number of large files in the folder, the indexing can take an eternity. If there is a corrupt file in the folder, Windows can't index it but it never stops trying.
One cure that usually works is to go into Control Panel>Administrative Tools>Services and set the Indexing service to "Disabled." I do that on all of the XP systems I maintain and haven't had any further problems with it.