Did you try changing the channel?
If you have your router set to automatically select a channel, it's possible that it selected a channel that your PC doesn't work with very well, or one that is experiencing more interference in the part of your house where your PC is. Antenna designs and internal sources of interference can make some channels worse for some machines and not others. Try manually selecting a channel and moving to the other end of the band.
Is there any chance your wireless router complies with European (ETSI) regulatory standards, but your PC's wireless card is a North American (FCC) model? ETSI has 13 channels in 2.4GHz, but FCC only has 11. If your router selected channel 12 or 13 this time, your FCC card wouldn't be able to work with it. Nowadays many cards are "world" cards that switch from FCC to ETSI mode when they have reason to believe they're in an ETSI-style region, but it's possible that some cards on the market still don't have that feature.
Even if you weren't using automatic channel selection, it's possible that a neighbor started using the channel you were on, and that the interfering signal is strongest where your PC is. The solution for that is the same: Try changing the channel.