If you live in an old house, that is, really old, your walls are probably made of a combination of hardwood sticks and a cement-like mortar; this is called ‘lathe and plaster’.
Tests on wi-fi signal strengths have shown that lathe and plaster walls are so dense that they reflect the radio signals that make up your wi-fi signal. The next worse thing to have in your walls between you and your router are metal ductwork. The metal of the ductwork also reflects the radio signals.
The solution is one of two things: buy a stronger router(or a booster for your existing one), or move the router closer to where you want to work.
If you go the new router route, you will want to buy an “N” router(that means 802.11n, Draft 2.0); these are the newest and strongest routers, designed to bounce the signal around obstacles.