Why does my PC reboot every 23 minutes?

Hard to tell, I think the events regarding Disk Defrag may be related, but could be read herrings. -

Press Windows Flag+Pause/Break, Click "Advanced System Settings" on the left, then click "Settings" under "Startup and Recovery" and make sure that "automatically restart on system failure" is disabled.

If you get a Blue Screen, post the error code here and we will help further.

If you do not get any Blue Screen, it means that it may be a heat / power issue or -

It could mean a bad hard drive and the Defrag is coming across a corrupt part of your hard drive, at that point it is causing the whole thing to fail - this could give a instant reboot without a fail code.

Also, it may be worth running a memory check on your system, just to rule it out.

Lastly, if it is EVERY 23 minutes, you may want to run a Linux live cd such as Ubuntu. If it does not fail within ~35 minutes, it could indicate a hard drive or another driver is causing you the problems.

That sounds similar to a problem I had on one of my computers running WinXP. I traced the problem to the "Zero Configuration Wireless" service that's supposed to find and automatically connect to the wireless network with best signal strength. The problem was if signal strength varied the service would often drop the service that was first connected and try to reconnect to another one. In some cases it didn't work that well and the computer would simply flash a blue screen and reboot every 15 minutes or so. I solved the problem by disabling the Zero Configuration Wireless service and instead always manually connected to wireless network I wanted.

I have no idea if this is your problem. But one thing to check is to see whether you get the same rebooting problem when you have your wireless network connection disabled.

I disabled the defrag task in the task scheduler (even though the history shows no sign of failure) and so far today haven't seen any reboot even though the system has been idle. Will leave for another day or two then re-enable the task and see what happens.

Tried running defrag manually but it doesn't seem to do anything - just quits and I see no process running in task manager.