Both ends are Tia568B ? or one end is?
Both the wall run (router -> jack) and the patch (jack to computer) should be the same
type of cable (both Cat 5, Cat5e or Cat6)
shouldn't mix cable types in a run
(standards say it's supposed to work,
but in my experience sometimes doesn't).
Do you know someone you can borrow a cable
tester from (loopback ones are best)?
Also what brand router? that can have some
effect.
Coincidently Tom's Hardware just put up a
networking site not sure how helpful that
will be tho.
http://www.tomsnetworking.com/