If your pc reboots playing guild wars, try un installing SP2. It worked for me on two totaly differnt machines.
It has made my laptop and pc both reboot time after time until i ininstalled SP2.
I had to share this with you as alot of people seem to have this problem. Have fun now ^_^
ya that's about the worst idea ever....let the software companies accomodate the operating system. SP2 offers exploit/security patches that are extremely vital to your machine. Downgrading your security just to play a game is an awful idea. Let NCSoft know that this has worked and allow them to discover what is wrong....and fix it.