Here are some hints for Sonic CD on the PC:
1) Collect 50 Rings on any level and then a giant ring will appear at the end of the level which will take Sonic to the bonus round.
2) To create a good future, go to the past and defeat metal Sonic and destroy Dr. Robotnik's evil creations.
3) If Sonic can collect the Gem on every level in each bonus game and finish the game with a happy ending, you will get the option to play in visual mode.
4) Sonic cannot die as long as he has at least one ring. Make sure Sonic is carrying at least one ring at all times.
5) Go to the future, present, and past to collect the most rings possible.
6) The future only affects the future, while the present affects both the present and the future, while the past affects the past, present, and future.
7) Some power-ups can only be reached in certain time zones.
8) In the Boss Level where Sonic must outrun Dr. Robotnik, run just ahead of Metal Sonic in order to stay ahead of Dr. Robotnik.