Internet Connection via NIC/Cable modem

I have an ASUS P4S8X mother board with an onboard NIC jack. The jack
supports 10Mbps and 100Mbps. Starting yesterday whenever I connect to
the net the little two monitor icon indicates that the connection is
at 10Mbps. Prior to Sunday I usually viewed the connection as
100Mbps. Has something gone wrong with the NIC RJ45 onboard
jack/chip(s)? Does anyone have a small utility that I could employ to
test if this is now a hardware problem and how to remedy.

Please post suggestions and a utility if you have zipped. I am
running Windows2K Pro.

TIA