i just completed the side quest where the reward is you become knight of the white stallion, and i somehow got vampirism, i did search for it on google, im just looking for the easiest way to cure it.
to let you know i have the xbox 360 game of the year edition.

Go to a chapel and pray. Or if you are too late for that (3 days), there is a quest that will remove it.

thanks

Another requires purchasing extra DLC's - Vile lair. An upgrade allows you to use salts from a mine within the room as an offering to a pillar also in the room to instantly cure vampirism (just used it)

If you play the game on a new character in the future - if you see Polyphonic Hemophilia pop up on your screen, pray/drink cure disease potion/cast spell within 3 days of infection and you'll avoid this.

http://www.uesp.net/wiki/Oblivion:Vampirism

If you have a save from before you got your first nightmare as a vampire, reload that and then get disease cured by regular means (chapel, potion, or mandrake root).

My characters are never without a few mandrake root in their inventories. Just check your current effects on the magic tab when you leave any vampire dungeon, and eat one if required (or in doubt!)

well today i was able to get all the ingredients to the cure vampirism potion, my khajiit knight (a good guy ) is cured.
even though it was a bit of a pain to do since the xbox 360 version is a bit buggy.