Many have wondered why you can't turn in some of the items required for the Cure Vampirism quest, namely Bloodgrass and/or garlic to the witch who gives this quest for the cure vamp potion.
If you have finished the main oblivion quest, this is the reason for the glitch!
It's to do with finding bloodgrass in oblivion gate areas and since you've closed all the gates, this is the reason for this annoying bug. Even if you buy it from vendors, it still doesn't solve the problem.
I have the same problem and was told by customer support to await a patch on xbox live!
Being stuck as a vampire untill Take-Two Interactive get their act together really sucks, and is ruining an otherwise faultless game