Donkarlito, your router is only wireless G, which doesn't give as strong a signal, or project the signal as far as a wireless N router can.
Look to cut out as much interference as possible, maybe try asking your provider to change the channel for you (they will do it over the phone and talk your through recconecting if you experience any problems) or look into the powerline adapters i suggested, these will cure all your wirless connection woes, and are great for providing a fast stable connection to your xbox/ps3 if it's not close to the router.
Another option is simply to switch providers. When i moved from AOL to O2 i got a noticeable improvement in speed, along with a free wireless N router for less than i was paying before. O2 get pretty good ratings from independent surveys for both speed and customer satisfaction.
