CBS last week reversed an earlier decision and will air a TV spot produced by the Department of Health and Human Services that discusses the new Medicare prescription drug law, a network rep confirmed. On Feb. 13, the network pulled the ad pending a General Accounting Office investigation into whether the spot contained political messages. CBS, like other major networks, has a policy against airing political or advocacy advertising. CBS learned the ad had been edited for content. It researched the ad and was satisfied that it contained no political message.