Hi simonsup, telling us to look at your earlier posts for the spec of your machine is all very good, but how are we to find your earlier posts? It would be easier for you to access your earlier post and cut and paste the spec to this one.
If your computer is busy doing something for 20 minutes, then you really need to find what is happening for all that time. I suspect there is a process started at boot up that is using all your computer resources. You need to look at the startup menu and disable as many processes as possible. It is also worth running Adaware or Spybot to see if there is any malware on your computer.