Have you been getting an intermittent error similar to the following during a TFS Team Build execution (within the CoreGet target) ?:

Access to the path 'C:\Documents and Settings\tfsbuild\Local Settings\Temp\TFSTemp\4580{#$@;._}808' is denied.

If you look around, you will find a few folks who point at antivirus software as the culprit, but in our case it turned out to be the disk defragmentation service…