After running long time (maybe 3-5hrs), my area-51 will slow down, the screen will be frozen. It is hard for me to run any softwares. I have to force it to shut down and restart it. Only restarting it, it will recover. But it only works for 3-5 hrs! What is wrong with my computer? Is there something wrong with my video card or i have to reinstall my operation system?
Sounds like it's over-heating. Use Thermal Controller to turn up your fans and/or run a util to monitor main processor and GPU video temps while gaming (to verify the problem).
Sounds like it could also be a software/driver issue. Memory leak or something like that. Try updating all your drivers and then run the system with the bare minimum backgroung programs running. If the problem goes away then run it with everything running in the background. If it then comes back then It's something you turned back on.