As most know, there are sets of channels (frequencies) designated for use by WiFi. In North America, the channels are 1-11. Other regions have channels 12, 13, and 14. These channels are set by government agencies such as the FCC in the USA. They are defined to make sure that there is not chaos in the EMF spectrum and devices aren't interfering with one another. Still, especially with 802.11n's MIMO, we've seen the 2.4ghz band (where all these 'channels' are) get very crowded.
In many places, such as apartment buildings, these WiFi channels are now all so crowded that people opting to use frequencies disallowed in their region. This caries legal consequences, so I advise NOT to do it! You can be fined or imprisoned.
Unbelievably, many times this is as easy as replacing the firmware of your router with an international version, then changing the 'region' setting. Most wireless adapters are created to work in any region, so will therefore do fine. While I don't have this problem, I have been aware of this for a while, and figured I'd bring it to the public's attention. If you do something like this, you are not only violating FCC regulations and who knows if there is something more important that you might be interfering with.
While it can be a great solution to wireless interference sometimes, a better solution is switching to the 5Ghz band. Sadly, these routers and APs are more expensive than those that use the standard 2.4ghz band.