one thing you may notice (it's happened to me a few times) is that the network cables dies, which is noticeable by their being no connection, which is easy to track down, however i have had one of the wires inside the cable break (only found out via a cable tester!), which meant downloads would stop and that web pages would part load before stopping, as Firefox was missing 1/8 th of the packets, and eventually decided it wasn't worth continuing to load
The moral? have a spare cable (or usb connection to the router if thats whats your router does) and plug it into another port on the router (could just be something in between the contacts) and try your pc on that before anything else, as if the problem is at your end then it is either usually quick to fix, or then at least you know what bits to buy!
[random]A friend of mine (who shall remain nameless) once had problems with his "Livebox", Wanadoo's name for it's all singing, all dancing wireless pyramid of joy which never worked for him. He called Wanadoo many times who assured him that everything should work fine, and the BT Line test showed no problems. Eventually he got so annoyed at "their appaling customer service" he thought "**** em, i installed the drivers and it worked fine, why don't they tell people to do that?" #sighs#[/random]