Dell PowerEdge T300 with SBS 2008 R2 Randomly Rebooting

Working on a Dell PowerEdge T300 server with SBS 2008 R2.

A few days ago the server began restarting itself over and over. I came in today and began troubleshooting - my first time ever working on this server, so I had to piece together the setup as I went along. Running Exchange 2007 and not much else - client basically just uses it as a mail server and DNS.

The first thing I noticed was a full C drive - totally full, 0 bytes free. This was causing all sorts of errors, as expected. I was able to free up 10 gigs from the C drive after a few hours of coordinating what could be deleted. During this time the server did not reboot itself.

Shortly after this the reboots began again - with increasing frequency. This led me to believe the system might be overheating, so I took the chassis apart and checked for hot components. Everything was very cool, PSU, CPU heatsink, the HDDs - nothing was even hot, let alone hotter than it should be. Event viewer doesn't turn up anything relevant - just a bunch of errors left over from the full C drive.

Ran a Windows Memory Diagnostic after that and it turned up clean. CHKDSK repaired a few corruptions on the C drive and booted normally, that's where we stand now. It appears to be running just fine at the moment. I disabled Automatic Restart on System Failure as well. I'm going to leave the server up and monitor its up-time remotely to see if that helped.

Any ideas? I ordered more memory because it's a cheap fix and the box could use more RAM anyway. Also ordered a new PSU which will be here in a few days.

Thanks for any help.