http://arstechnica.com/guide/networking/installation-4.html
Fishing is the term for guiding wiring into existing internal walls
Google on "installing Ethernet wiring"
The two essential tools you may have to buy or rent are a long drillbit (16" - 24" long) that allows you to drill through the sole plate of a wall into the cavity of the wall below.
and a Fish Tape - a reel of spring-steel strip that's flexible enough to go around corners, yet rigid enough to push and guide from one end. It also typically has a hook on the end of it, so you can use it horizontally through a wall plate, to catch a hanging cable coming from above, and pull it out through the wall plate. If you don't have a fishtape, a modified wire coathanger can pinch hit on the second task.
For horizontal runs, you can often get Ethernet cable jammed into the gap between the carpet and the wall-floor moulding or baseboard. You don't usually need to open the wall to drill down to the lower floor, you can go in at an angle from the corner of the wall. If you can remove the moulding to do this, then replace it when done, it'll be invisible. If crossing the floor is impossible, you can run it along the corner of the ceiling, or up and around doorframes. Leave yourself beaucoup length of cable to handle pulling wire and making these routings. CAT-5 is good for about 1000 feet, so don't sweat an extra 50 feet. Just don't nick or crimp the cable.
Hint: When you do pull in a cable, along with it also pull in a length of sturdy nylon twine. Then, when you find out later you have to pull a second cable through the wall, you attach the cable to one end of the twine and by hauling on the twine, you pull it through in jig time.
But then, you protest, you don't have your twine anymore? Simple, when you pull through your new cable, you also send another length of twine at the same time.