Street Fighter X Mega Man Cheats

All Eight Bosses, 4 E-Tanks
At the title screen, select Password, then enter this:

All Eight Bosses, 4 E-Tanks: A5, B2, B4, C1, C3, C5, D4, D5, E2

Start new game with 4 E-Tanks (Version with Password only)
Go into password menu, then enter this code. You will start a new game with 4 E-tanks.

Start with 4 E-tanks: A5, B1, B3, C4, D2, D3, E1, E4, E5

Guile's Theme
Pause the game in any level, then hold Jump (default L) and press U D D D (default keys W S S S) to have Guile's Theme replace the background music until you put the code in again.

Hidden Abilities Code
Have the ability to use Hadoken manually without any meter or defeating Ryu.: Press and hold the jump and fire buttons for about 5-6 seconds.

Play as Mega Man without his Helmet
To play as Mega Man without his Helmet, hold shoot on Ryu on the stage select screen and then press Right Right Right Left. If entered correctly, a sound will play and Mega Man will lose his helmet on the stage select screen.

How to get an easy perfect
Just before killing your stage boss, use your E-tank to refill your health back to maximum. Doing this will trick the game into thinking you've earned a perfect in the round. Once you kill your boss when your health is full, you will earn a Perfect.

More E Tanks
If you need more E tanks to beat the game. Just let the timer after you die hit 0, go back into the game and you will have all the tanks you previously collected and you can recollect the E tanks from the levels all over again. I got 10 E tanks at one point.