I just bought some ENC28J60 Internet Shields and hooked one up to an Uno, swapped the libraries, loaded one of the Example programs under Ethernet
You have to explain, in a lot more detail, exactly what "swapped the libraries" means.
You should NOT have deleted or replaced ANY of the libraries in C:\Users\Host Guest\Desktop\arduino-1.0.1\libraries. That is where the Arduino-provided libraries go. Though why the IDE was installed on the desktop is mystery.
User downloaded libraries, such as WebClient and EtherCard (the library for the ENC28J60 cards) go in the libraries folder (you may need to create it) of the sketch directory.
The messages in the last post indicate that Client.h has not been updated for post 1.0 use. The messages are pretty explicit about what needs to be changed.
Client.h:22: error: conflicting return type specified for 'virtual void Client::write(uint8_t)'
The file name is specified, the line number that the error is on, and exactly what the problem is (void should be size_t). You'll need to edit the .h file, and the .cpp file, to change the type, and to add a return statement to the function (return the number of characters actually written, usually 1).