The easiest way to go from a pure evil character to a pure good character is to counteract the two things that make you evil - corruption and impurity. Certain foods, like the Tofu, will raise your purity: check them in your item menu to see which ones can help. Stock up on these foods, and eat them when you can; if you eat them too quickly, or too much at a time, your character will grow fat.

If you own properties, set the rent for them as far down as possible - this also counteracts your corruption, and will help you become more pure. Do as many quests as you can with the "good" character options to gain large bonuses to your purity.

If you are skilled at Blacksmithing, you can become a pure good character very quickly - play the Blacksmithing mini-game and make a large amount of money, then donate all of it to the Temple of Light. Repeat this process(it may take a bit, but not as long as completing multiple quests) to gain large bonuses to your purity in no time.

FIRST BUY ALOT OF PROPERTIES. IF U ALREADY OWN PROPERTIES, SET THE DATE ON YOUR 360 UP A YEAR B4 U PLAY NEXT TIME. THEN LOWER THE PRICES ON ALL YOUR PROPERTIES AS FAR AS THEYLL GO. LOWERING THE PRICE OR RENT ON YOUR PROPERTIES IS AN ACT OF PURITY.

Fable 2 is so in-depth; you can swing from bad to good in as easy as one or two decisions. Since you became bad most likely doing multiple bad deeds, or possible decided to go Anakin Skywalker on an entire town ("I killed them all"). As easy as it was to loose those good points, it is as easy to earn them back. Since good is always a longer more difficult road, becoming 100% good takes time. Keep doing good deeds and helping where you can. For a quicker result, but also more painful, find a worthy person to give all of either your money or items to.

Note: Might want to save your game before you do so, just in case you change your mind.

The answer posted above my own is another example of a way to accomplish it.

 

A few ways this can be done include attracting a large crowd by playing lute and then eating tofu, giving gifts to villagers, using friendly expressions and lowering prices or rent in shops and houses you own. The fastest way to become good is by finding a beggar and giving a large ammount of gold to him.