Sonic & Knuckles Collection is a complication Sonic 3, Sonic & Knuckles, and Sonic 3 & Knuckles all on your PC. The US and European versions also contained Sonic the Screen Saver, which can be used to dazzle your desktop. Japanese fans, however, didn't get the screensaver with their version of the S&K Collection, so they had to buy it seperately.
Although I have only played a demo of this game, I'll review it anyway since I have S3&K for the Genesis, and it's almost exactly like the PC version.
Those who can't get a Genesis now have their chance to play this great game! The 16-bit music you hear in the Genesis version has been re-sequenced into MIDI files, which I think sound way better, and the graphics look somewhat enchanced. If you don't have a Genesis, but you have a PC, and you like Sonic, get this game!