I had the same problem before

I am kind of sure that it is due to virus. Check the running program behind your screen. You might find application(s) with the name of nwiz.exe or ndomen.exe (I can not remember clearly about the spelling of "ndomen.exe", but something similar. Also there might be other worms. Just check with application is consuming your memeory and CPU). Restart the computer and you will find it starts to eat memory continuously and your CPU usage will be 100% all the time. You will also find that your Norton AntiVirus can not do the LiveUpdate anymore. To solve this problem you can try to search in google. Since I was reinstall the system when it happened I just format my dard drive. If you do so, remember to install your antivirus software first and keep it update before you connect your computer to the internet. It is also strongly recommend you save Windows Update Patch to a CD and update your system before you connect to the internet. Anyway, I think you will be fine even you update your system online. Just be sure to install your antivirus software and keep the virus definition up-to-date before any further action. Good luck.