Desktop wireless suddenly broke down

Hi. I have a BT home hub wireless network which works brilliantly everywhere in my house - on our laptops, iphones etc. But my Inspiron 530 desktop has stopped connecting to it, so I have no internet connection at all on my main computer. It was working fine one minute (this is several days ago now) then started getting slower and slower until the download speed crashed to under 50 kbps and then stopped. now it can detect the network but can't join it. None of the networks diagnostics suggestions seem to make sense... eg. it can't be too far from the router if it hasn't moved, and there can't be interference if nothing else has moved.
the laptop I'm on and our other bits of kit are unaffected so I'm inclined to assume it's a problem with the pc rather than a problem with the network. any ideas anyone?

many thanks in anticipation...

C