To help ensure it is working properly, make sure TCP/IP is installed and working properly. Pro/E doesn't work well with DHCP so you may need to get a fixed IP or get a router and set the router to dhcp and set your IP address to 192..... LAN address.
What kind of card is it?
On Win2000 you can open a DOS window and type ipconfig /all and it will report the network information. Look for the line Physical Address and make sure it is filled in, this is your cpu id.
I have win 2000 but I'll try to get you there. Look for Network settings. I right click on My Network Places and select properties. Then I right click my LAN connection and select propertie. Select TCP/IP and select properties. Check the Use The following IP address radio button and enter an IP address. Use for example 192.168.100.101. Enter 255.255.255.0 in subnet mask and 192.168.100.101 in the gateway. Reboot and try to install. Yours may be slightly different. This should set it up for a fixed IP.
This problem is descibed in the technical support on the official PTC website. Search for 'ethernet' and you will find the answer.
New windows versions are trying to 'turn of' the network adaptor when it is unplugged. Two possible sollutions: Enable the card in windows by a right mouse click and choose 'enable'. The second sollution is much more complicated where you have to edit the registry of your workstation, see http://support.microsoft.com/support/kb/articles/Q239/9/24.ASP
Definitely reccommend setting up a loopback adapter (google it, there are some very simple guides) before installing any PTC licensing. Disable any ethernet connections (including wifi) before installing and use the mac address of the loopback adapter (from cmd> ipconfig /all) Thanks ViZ for the post.