Is My Network Card Borked?

[Exonerate]

Yesterday, out of curiosity, I installed Ethereal on my computer and took it for a test run... and to my surprise I was getting many, many errors (I'd say about 50% of my packets were getting TCP checksum errors) and obviously, this isn't good. Furthermore, my packet sizes were all very low - about the 300 byte range, and I know that my connection supports more. I have my MTU set at 1500, so my packets should be something closer to that size. I disconnected my router from the modem, then did a little testing between my computer and router, and found that the errors still persisted. So. This narrows the problem to one of the following:
My software
My network card

Anyways, I plan to do some testing later to see if I can eliminate the router as one of the choices... Does anybody know of any possible situations that would account for the errors I'm getting?

Edit: The router has been eliminated as one of the possible problems - after directly connecting to the modem, I am still experiencing the same problem.
Edit Again: The ethernet cable as the source of the problem has been eliminated as well - I still get the errors with another cable. So it looks like it's either my built-in network card or something else...