为什么删除的文件会重新出现在Vista中？

I have an automated test running on my program that generates some large MPG files along with other files. After running the test, I have the script clean up after itself. The other files (a mix of binary and text files) are removed with no problems. The MPG files, however, don't get deleted. If I try and manually delete the MPG files from explorer after the test has run, they are removed from the explorer window only to reappear a few seconds later. Process explorer shows that they are being used by explorer.exe. Eventually, I am able to remove these files. Any idea why this is happening?I have an automated test running on my program