Removing a smart virus

My Old desk top computer got a smart virus. I know how to remove most viruses. worms. rootware ,ect . But what i have on this is too smart. every time i try a virus scan with windows security it will cause my computer to restart it self. Even if i try to download something like AVG its will get close to finishing downloading and then the computer resets it self. Its preventing to set the computer to an old restore point and a rebooting. it even prevents me from sometimes even going into safe mode. Most computer places tell me its gonna cost me $300+ to remove this virus. I can buy a new one for that much. But i don't have the money to buy a new one or a good use or to fix it. What am i to do to remove this smart virus?