A simple login page on Arduino Uno
I have a page about a tiny web server for the Arduino Uno or similar boards. It simply isn't true that you can't make one, or that a much more powerful processor would be better. There are some limitations imposed by small amounts of RAM, but the library described there decodes incoming HTTP information "on the fly" so it can handle a reasonably large page.

You could put the data to be display into PROGMEM (more information here) or onto an SD card as someone suggested.

The login and password could be stored in EEPROM.