The symptoms you describe could be viral related or they could be due to hardware or overheating problems caused by a failed processor fan, bad memory (RAM), failing power supply, underpowered power supply, CPU overheating, motherboard, video card, drivers, dirty hardware, etc. When was the last time you cleaned the inside of your computer?
Dust restricts the airflow and prevents proper cooling. This in turn can cause overheating and faulty processor fans which can result in unexpected shutdowns, random restarts, booting problems, etc. Open your machine and make sure the fans are working and the heat sink on the processor is not blocked with dust or debris. Remove the CPU's cooling unit and clean the fins on the heat sink that sits under the CPU with a can of compressed air. You can also remove the cards, RAM modules, clean the contacts and reseat them. Check your connections and fans to make sure they are working properly and monitor the temperature. If the computer is overheating, it usually begins to restart on a more regular basis. See "Cleaning the Interior of your PC
" and the "Illustrated How to Replace an AGP Video Card
" (you can also use it as a guide to clean & reseat).