thanks all,
one day before it's running well. and as long as i remember i did not put new software or hardware in my computer.
yes, it's reboots exactly berfore login screen.
i'm using compaq desktop pentium IV with memory 256 Mb (all hardware still standart, vga and network card ,sound card on board) my computer connecting to the network. my server always update service pack for windows and mcafee anti virus, i don't think this is the problem because we have couple computers like mine in the same network.
i did f8 tried safe mode but after it's finish start up... before login sreen it's reboot again, and i did Last known configuration still no luck...