Try installing XBMC on the old laptop (XBMC.org) and use that to connect to your PC. The problem with the image not filling the entire screen is DEFINITELY NOT because the of signal strength in your wifi. It more likely has something to do with the laptop video resolution, or the aspect setup of your TV (Normal, Zoom, Panel, Stretch, and so on.)
If you want a tidier solution (and one that comes with a remote control), the Dune boxes that DesignGuy suggests seem to be good, or the Western Digital boxes (which I don't have a prejudice about. I've used 'em. They work.)
I wouldn't recommend the Roku (I have one of those, too) because its streaming capabilities are tied up in running a server on your source PC (Plex or Subsonic) and it's a little clumsy in operation.