need a bit more information, but in theory it shouldn't be 'impossible'
If your modem is a USB modem, then you will need to network your Wii through your PC, if your PC has no ethernet ports then you will need (2) USB to Ethernet adapters, (one for the Wii, check it is compatible before you buy) then plug the wire from PC to Wii, and go into network places and run the (setup home or small office network) network wizard, allowing internet sharing through your (new) LAN connection.
Check you get the right cable as well, you will need a cross-over cable I believe (J42 springs to mind but don't quote me).
Much easier than all ^ that..... buy a Nintendo WiFi USB Dongle, it will send the internet from your PC to your Wii wirelessly with minimum hassle, (they are fairly easy to setup and come with instructions) also Nintendo's website will tell you how to do it this way.
In short, unless you have equiptment that interferes with WiFi frequencies, don't bother with a wired connection, it will cost you more and be harder to set up.