A win2k8 server is running out of disk space but I cannot delete any files

I was doing a TFS long haul stress run on a win2k8 server this week and the server ran out of disk space. While I was trying to free up more space on the server, I ran into this issue:

1. In Windows Explorer, delete some files. These files seem to go away

2. Check available free space, it didn't show any increase.

3. In the Windows Explorer, cilck on refresh, these "deleted" files are showing up again.

I tried to bing this issue but found no hits. I eventually just rebooted the machine and it took a long time for ther reboot to completes. But the good news is that after the reboot, I'm able to deleted unwanted files again on this Win2k8 server to free up more spaces. I have not figure out what's the root cause for this behavior yet though.