Did you perform your speedtest while connected via wireless?
Wireless is shite, just accept that, plug a cable in a deal with it.
But yes, if you're on 25Mb, and you're getting 12, that's pretty good. Shitty as it seems, that can't really advertise it any other way, they can know the speed at the exchange.....after that you're at the mercy of the dodgy green copper under the road.
Colin's diagnostic procedures are spot on, follow them.
But really, lose the wireless, that's probably why you're slow.......not because it's actually a slow wireless connection, but just because everything needs to be sent 75 times over and over again, because next door have a microwave and a baby monitor.
I'm only 8 feet and a weetabix wall from my wireless access point.....and it can take a good 0.5 seconds for my hitting the spacebar, to pause a video I'm watching, if connected via wireless.....so I am perpetually cabled up.
Setting a password can be tricky....best bet is to have a chat to the router (usually they have a webpage as a control panel, becuase they run their own operating systems, usually FreeBSD(or OSX as Mac users call it), or a very slim Linux.....can't scale you see....or for that matter....run 24/365. You have to make sure the router is set to use the same type of encryption as you want to set on the computer.
Best security though is a nice, solid, relatively leak free, ethernet cable, less that a fiver from a reputable computer supplier (or £45 from PCWorld).
I mean...last time I seen your PC, it didn't look to portable, so why exactly would you need wireless? Sometimes I think people decide to use it becuase it'd tidier, which is a fairly stupid reason to ruin your internet experience.