First of all, are you using high quality patch cables that are certified for
100-Base-T?

If you buy your patch cables, I would reccomend that you buy patch cables
certified as Cat-5e.

If you install the Intel Proset software that came on the CD, you can bring
that up, and try changing the speed setting there. It normally defaults to
auto-set, but you can manually choose your speed.
I have a few store bought cables, but I usually make them
myself from Cat-5e cable. I know they work, because the router always
detects them at 100 and I can transfer files a lot faster than 10
mbps. I tested it with this new NIC, and it's taking 15 mins to
transfer a file that used to come across in 3 or 4 mins - so the
software's not misreporting the speed or anything.