I am assuming it does this to you when you aren't watching... they tend to do this. It isn't normal for it to lock up every 3-4 days as I have had a W2K T-Server running for 4 weeks now without restarting it. It is possible that your memory is damaged, or possibly something in your MB. The other possibility is environment. After running into massive problems with another server I have been maintaining, I finally decided to leave a hardware monitoring program visible on the screen. Unfortunatly, I had to leave the system logged in to accomplish this, but what it did is show me that the system was suffering from internal brownouts, a faulty power supply, as well as the temperature had climbed into the 70C range. So, I cooled the system using very inventive methods and replaced the power supply and I haven't had a problem with it for over 3 weeks now. So, bottom line, your going to have to start poking around. Also, monitor the incomming requests to your server, it is possible that someone is targeting your system with requests for the TS service.
Hope this helps.
Cryptospy