How to check if a processor is working properly?
 

I suspect there's something wrong with the CPU of my work pc.

If I multitask, it freezes. And by multitasking I mean opening a simple accounts program and excel at the same time, because this pc isn't used for gaming. Even in explorer, if you click for example a folder it spends like 5 seconds 'searching' before it shows the files. The mouse pointer is 'sticky' too, menus 'stick'... Whatever I click takes 2 seconds to show on monitor, even if it's just clicking to choose the type of notification I want for this thread!
It's working like my dad's 1.8Ghz Pentium 4, only this is a 1.86Ghz core 2 duo. This problem started around a month ago, but it was more like it was gradually getting worse, it didn't kinda appear overnight.

I've scanned for viruses, for spyware, defragged the hard-drive and optimised it, defragged the registry... I dunno what else I can do? It's a relatively new pc around 11 months old.

Here are the system specs:

E6320 @ 1.86Ghz
1 x 2GB RAM
Some asus mobo & onboard graphics
OS Windows XP SP3

I was suspecting that the processor was working on only one core, because it's exactly the same as a normal P4... but the task manager shows that both cores are working?

What do you think?
http://img.techpowerup.org/081120/untitled.jpg

I wouldn't imagine that its a hardware problem you have. Download orthos or prime 95 and run them for a couple of hours to make sure.

I would be leaning towards software. Is it possible to format the hdd and start over???

Assuming your computer is clean from infection, I would look towards hardware problems. As DrunkenMafia stated, run tests to determine if this is the case, or if something else is the cause of the problem. The first thing I'd check is your ram