Follow the linky mate...F-secure is currently £26.95 for a year for a single pc...you just download it once you have paid for it.
Overclocking is a bit of a 'black-art'. Essentially, any computer (be it an iPad, PC, Xbox whatever) pushes voltage around its mainboard in cycles. The voltage is either on or off to represent a bits status...one or zero...binary, the basics of any computing. The more cycles you can complete in any given amount of time the more work can be done.
Cycles are measured in hertz (so 1 Megahertz is 1 million cycles per second, 1 Gigahertz is 1 billion cycles per second). The Intel i3 4130 3.4GHz specced on your pc build is therefore running at 3.4 billion hertz per second...this is why you need a cooler sitting on it with thermal paste to aid the heat transfer...without it your pc would constantly shut down to prevent the processor turning itself into a molten heap
Both the processor and the memory have to match up the cycles used and to do this they have a little heartbeat called the clock...kind of like a metronome, it allows the timing of the cycles to synchronize.
Overclocking basically allows you to tweek the clock cycles to make the processor and memory speed up and therefore increase the speed and efficiency of the system. There is a down-side to this though. Pushing the processor, memory and mainboard beyond their limits causes heat and stress...finely tuning the system to get the best performance without frying everything is the 'art' I mentioned.
Personally, I have never bothered with it - it leads to a whole sub-genre of pc modding and cooling. I can play MW3 quite happily on my screaming death machine without tweeking a thing