How do you cure being a vampire in the elder scrolls IV

There is a long quest you need to complete to get cured. (see related link)

Note: There is a major bug on the PS3 version if Shivering Isles is installed (either because your playing the GOTY edition, or because the Shivering Isles expansion pack is installed) that will result in you not being able to be cured and you will be permanently stuck as a vampire. See the bugs section of this link for more info and workarounds.