Oooh then it can't be the hardware. I'm assuming this didn't happen at first? Probably has to do with some new software. Installed something new?
Did a proper virus scan and everything?
By network and download speed, do you mean that's the speed your provider is supposed to give you... or that's how much you're actually downloading at?
I usually get fed up and reformat everything, but that would be the absolute last thing.
12 Mbps is what I'm actually downloading at (tested at speedtest.net). I just ran Avast and it didn't come up with anything. I don't remember installing anything new.