The machine i use for a router and firewall is running windows NT4 and it gets left on 24/7, in the last 3 months its only been rebooted a couple of times for updates and i`ve never had no problems with it. It just show that not all Windows boxes are unreliable, i personally think it comes down to the hardware and what its used for more than anything.
I don`t leave any of my workstations on for more than a few days at a time because i tend to find they get a bit clogged up with diffrent applications being opened up and closed all the time, sometimes you can notice a quite a diffrence after a reboot.