Laptop ok but pc wont connect

Hi guys
I have a wireless network through Virgin Broadband which consists of my modem connected to a Pluscom WR-AR2317, one laptop (which belongs to my work) running XP and my pc, running Vista. My laptop connects no problem and my friend set up the wireless modem which worked a treat at first with both computers. However over time the connection to the pc has got weaker and now it will not connect at all. The pc states there is a weak or local connection but will not allow me to surf the internet at all. I have a feeling it is a lack of signal strength as I live in a large old house and the router is in my living room, whilst the pc is in the back bedroom. The pc has a Belkin wireless card in it (not sure what model). What I can't understand is that my laptop connects regardless of where it is, even if it is next to the pc! I want to resolve this problem and I am willing to shell out for new equipment but I don't know whether to replace the router or the wireless card. I am also confused by the whole N or G debate as I can't tell what I have at the moment.
This stuff is so tricky!
Thanks