I think I agree with CNC here, having *that* much would be a problem: Therefore I'd build myself a nice place to live, buy all the toys/ gifts I wanted, travel for a few years and make investments to set myself up comfortably for life.
Then I'd like to devote the rest to some kind of good cause. Rather than just give all the cash to charity (although I'd probably give some), I'd like to set up something that I could have a hand in running myself, and that might actually turn into a viable business concern:
-Set up massive renewable energy schemes and sell the power to the national grid.
-Create usable and profitable urban mass transit systems (something badly needed in this country)
-Fund a martian colonisation project
-Fund development of one of those orbital passenger planes. (Hotol etc)
-Invest in airship technology (haven't you heard? Airships are back!
http://www.cargolifter.com )
-Invest in affordable housing development on brownfield sites.
-Invest in hydrogen cars and fuelling stations.
-Buy Microsoft and put the Windows code out open source
Oh, and I'd buy Malfador, pay Aaron a huge salary and let him employ loads of programmers to help him get all of our wish list items into SE5=-)
Of course it's easy to say all this, but in reality I'm lazy so I'd probably just sit on the cash and slip into the billionaire lifestyle.