I know the advert is based around Internet Explorer rather than Microsoft as a wider organization, but that level of misleading advertising has to be in breach of trading standards (in the UK our trading standards laws are pretty strict).
While that argument might stand up in court, Trading Standards and ASA (Advertising Standards Authority) do take misleading consumers as seriously as outright lies.
The real question is whether they consider the PRISM leaks as evidence to challenge the tag line. Given the secrecy of the operation and Microsoft's denials, it the standards authorities might consider that evidence too much as hearsay to take definitive action.
I know the advert is based around Internet Explorer rather than Microsoft as a wider organization, but that level of misleading advertising has to be in breach of trading standards (in the UK our trading standards laws are pretty strict).
[1] https://www.youtube.com/watch?v=bt51MWll1oY