In this scenario, are we getting the money right now, as in today? If so, I'd take a chunk of it and do everything possible to make sure my dying grandfather is comfortable for however long he has left.
Give $1 million to my parents. They're fairly well-off, but a large portion of their retirement fund was shot to hell by Bernie Madoff a few years back, and they haven't fully recovered from it.
Everyone here is saying they'd build a house. I don't really want to build a house, myself. I live in the city, so that's not really a viable option, and I don't want to move somewhere else since all my friends are in the area and my family's only an hour away. What I would do though is buy an apartment in that beautiful complex a few blocks from me. Two of my friends are living there, and it's quite lovely. Maybe also buy my current building from the landlord. Three nice (albeit a bit small) apartments in a beautiful neighborhood (and literally right next door to a school) is a good investment. Maybe invest in some other real estate, make a consistent flow of income from renting things out, so I don't end up with this eventually running dry.
I'd consider a car if the public transit system around here wasn't so thorough. It is, though, so a car would be unnecessary.
Definitely take $1,000 or so and build myself a powerful new gaming rig, something that will last me several years.
Set aside a few million just in case. Good to have some emergency money in case anything happens.
After all of that is settled--once I have a place to live, a decent computer, a steady income from real estate investments, and a sizable emergency fund--use the rest to start up a game company. By which I mean both video games and board games, since I have plenty ideas for both but can't feasibly make them a reality on my own.
I know I may be coming across as a bit selfish, not giving any of this $100 mil to charity or anything, but I'm rather fond of this Roman proverb: "medice cura te ipsum," or, "Doctor, heal thyself." One should make sure they're secure themselves before they put too much effort into aiding others. Once I have a more steady, sizable income going (hopefully from the game company), then I can start with the philanthropy.