My cruddy old garage computer pooped out on me. Download speeds were 1/10 normal, while up speeds were normal. After a little troubleshooting, I determined that only that one computer had the problem, so I swapped out the NIC and got NO connection.
So out of frustration and uncertainty, I pulled the HD out and slammed it into the computer I had in my bedroom - along with the HD it already had in it. Booted it up and BANG! Problem solved.
Then I took it out to the garage and ...
... no internet ...
My mistaken assumption was that since other computers running off of the switch in the garage were unaffected, the switch wasn't the problem. Turns out I have one bad port on that switch. So I moved the ethernet cable to an open port and all's well. And now I'm running a 3 GHz dual core instead of a 1 GHz P3.
I tried moving a Windows HD to a different computer once.