Is there a known issue leading to file modification times of cache files on Windows XP SP 3 getting arbitrarily updated, but without any actual change?

Is there some service on a standard Windows XP - Backup, Sync, Versioning, Virus scanner - known to touch files? They all have a .txt extension.

If there isn't, forget it. Then I'm getting something wrong in my cache routines, and I'll debug my way through.

Background:

I'm building a simple caching wrapper around a slow web site on a Windows server.

I am comparing the filemtime() time stamp to some columns in the data base to determine whether a cached file is stale.

I'm having problems using this method because the modification time of the cache files seems to get updated in between operations without me doing anything. THis results in stale files being displayed.

I'm the only user on the machine. The operating system is Windows XP, the webserver a XAMPP Apache 2 with PHP 5.2