Here is what I would personally suggest, having encountered a similar scenario once a long time ago...
Find a place to park about $4.9M in a CD, Money Market account, or similar ultra low risk location. Don't get too hung up on getting the absolute best interest rate, just find a place that will give you easy liquidity (ie: DON'T make a 5 year investment right now).
Then, buy and read the following books:
"A Random Walk Guide to Investing" - Burton Malkiel
"The Only Investment Guide You'll Ever Need" - Andrew Tobias
(and/or any equivalent titles that catch your interest).
Then once you have a good/better understanding of financial investments interview some financial advisors and talk to them in depth about how they plan to manage your assets. Ideally you're looking for something more along the lines of a "wealth manager" vs. a financial advisor.
Killing a year finding the right investment approach while your money makes even a very low percentage rate is, for a 20 year old, a better approach than rushing out and making possibly a bad investment that incurs a big loss, or has tax liabilities you don't fully understand.
In the shorter term, you should find a good local CPA-for-hire and make sure you understand any possible income tax issues. In some (many) cases, if you will owe a large income tax return you will need to pre-pay or make estimated payments. Not doing so can take a big bite out of your ass later (ref: portion above about keeping funds semi-liquid).
BTW, congrats on your success. Take it slow and don't be tempted to believe you know what you're doing or can repeat this easily (learned advice ;) ). You have much time and luxury to plan your next move, and no matter what anyone tells you, you are not missing out on the investment opportunity of a lifetime in the next 24 months and do not need to make any rash decisions.
At the same time, you should sort of off-handedly tell family and friends that you locked in to a great investment and in 5 years or so will be able to access some of the funds. This proactive approach is better than having to say no to all the outreaches for "help" and "personal investments" that tend to come with a large chunk o' cash.
Whenever these types of questions appear I find them a bit hard to believe. The anonymous OP suddenly has $5M in cash? There's just a un-cashed check for $5M hidden in the sock drawer? Where is the money now? As soon as you show up at a bank with more than $100K you're going to be swarmed with people suggesting various things to do with your money. Hasn't the OP already had to deal with all the tax and legal surrounding a transaction like this? He or she should be familiar with a flotilla of financial professionals by now.
Personally, I think you'd just be better off buying government bonds from your favourite world leaders.
If you tell a bank you have $5m to to put in CD's they'll wet their pants alright, but it won't be security they call. It'll be their boss to gloat. (Unless maybe you bring it in in $100's in a duffel bag.)
That said, there's plenty of places you can invest in AAA securities that pay much more than 1%. Split across several of these the capital should be safe until he finds a better place for it.
The fact remains there are a lot of simple, understandable securities which the ratings companies are across and have a long history of payments to look back on. I'm talking about corporate bonds where you can assess the creditworthiness of the company, government (non-US) bonds of stable and credit-worthy nations that pay much more than 0-1%
I've come to the opinion that there really is no way around getting a little bit savvy about investing oneself. Until then, I would probably also stick to something fairly conservative. Or at least decide on a basic split into risky and less risky investments.
Even Graham has said that for most people a low cost index fund is the way to go. Competing against a computerized/connected/inside information Wall Street is very hard and because of transaction costs (higher to the individual than the institution) winning stock trading strategies are non-trivial.
However, Claude Shannon (famous computer science guy) was a pretty good investor: http://en.wikipedia.org/wiki/Kelly_criterion
Fortune's Formula http://www.amazon.com/Fortunes-Formula-Scientific-Betting-Ca... details some of Shannon's methods and covers some other interesting stories like LTCM, Mathematician Edward O. Thorp's black jack schemes and later his creation of the one of the first "computerized" hedge fund.
I found this edition helpful in that regard: http://www.amazon.com/Intelligent-Investor-Definitive-Invest...
Each of Graham's chapters is followed by another chapter of interpretation/reflection on his advice in a modern context.
The fact that the HN readership seems to be aggressive in the former case and conservative in the latter is not necessarily a contradiction (or, to be honest, even surprising).
Thus, we may be thinking of it in terms of "how can I use this money to invest such that I can live off of the interest for the rest of my life, getting the equivalent of a decent salary but without the actual work"? If I got millions of dollars, this would be the first thing I would be thinking about, not about how to risk it all just so I can be another greedy douchebag unhappy with their single-digit millions.
The "conservative advice" is intended to buy him time so he can figure out what he should do.
Note that startups aren't about seeking risk. They're about exploiting cases where the perceived risk is greater than the actual risk. The advice givers think that his perceived risk is currently lower than his actual risk. They're trying to reduce the latter.
So if you get $5M you should be looking to sacrifice a year from your life? Doh!