At NESBox.com (http://nesbox.com/) you can play any NES and SNES game, including The Guardian Legend!
You can play the original japanese version (http://nesbox.com/game/guardian-legend-the/rom/7e0dfd861704758dafe9e103cac106fb), the american version (http://nesbox.com/game/guardian-legend-the/rom/cb8ff92677e401510dd4fda2caba6c5d) and there's even a russian fan translated version (http://nesbox.com/game/guardian-legend-the/rom/6f405caad0d14091f80062d0ff446c31)
There is great Java browser emulator(and absolutelly legal, that's why not all of the games there):
http://www.virtualnes.com/
It runs very fast(no lags even on pretty old PC) and has stereo sound.
Here is Guardian Legend:
http://www.virtualnes.com/play/?id=NES-GD