I have nothing running at startup , all useless services disabled, it's defragged regularly and zero spyware
The answer is somewhere in there. First off, excluding Windows, you do have 'Startup's. AV, spyware, WB etc. My reference to ZA was not the firewall, but the apps that run with it. Secondly, everyone has their own opinions as to what 'useless' is and it's impact on speed, but very few actually hinder performance. Disabling some could actually have the opposite effect.
Your machine should have no problems running anything. Having said all that, my money is still on a security app that is on-access. Most likely a spyware/phlishing type. When you hit a site (as in forums) that have a lot of frames, scripting et al, your CPU instructions are off to the races. On a dual processor, it's not a problem. On a single, you're done until it's done. Since you have a dual, it's not CPU related (unless you've changed the affinity or priority), so you either have a service disabled that shouldn't be or it's app related. My guess is that your CPU, as it relates to your fan problem, is spiking more then you think.