Masive usage of CPU

You can add RAM to your computer. Auto-scan your computer at crucial.com Uninstall your anti-virus programs. Go to control panel and then to administrative options open services and end the ones you don't use.

How many anti-virus did you install? You can only install one or else they would conflict. Also, anti-viruses eats a lot of system resources.

Definitely download Spybot Search & Destroy. Also try CCleaner, and run the registry check as well as the cleaner.

You can add a memory because usually PC support 2 or 3 memories. It is a efficient way to accelerate your PC. And you can turn off the anti-virus software after you have scaned the PC, anti-viruse will slow your PC greatly because it will check every web page. And you can clean trash files often to make the PC run quickly.

i think u have effected with a spyware. do one thing instal spybot. and update it. check for any problem in spybot. it will display if u hav any spywares. try it

Difficult Without being able to work on your pc, it's very difficult to determine the root cause of the problem. "Services" is what controls all other "System/Background" applications that runs as a "Service", which is why your pc reboots if you kill it. You can see which services is installed and running on your pc by going to control-panel\Administrative Tools\Services. From here, you can stop unwanted applications. Just be carefull tho as a lot of these services is vital to the functioning of Windows and the ability to connect to the internet. The major reason for these kind of "Unexplainable" slow downs, is Spy-Ware/Mal-Ware/Add-Ware. It might be that your anti-spyware software is not aware of the threat or has previously "half cleaned" a spyware application. There is a lot of anti-spyware software out there and some of them are in actual fact spy-ware themselves! Apart from spy-ware, it can also be that your registry needs to be cleaned, due to installing/uninstalling a lot of software and even due to the cleaning of a virus/spy-ware application by your anti-virus/anti-spyware. The most basic thing you can do, is to empty your Temp directory and do a "Defrag" on your hard drive...let me know if you need help on that. Other than that, I can't give much advice without being able to physically work on your computer.

check again you PC by using good antivirus like kaspersky, and also check your PC registry fixer (like windows advanced care) and also make sure you have space left and enough RAM. Good luck