.. What do I do with the deck of cards?

You won't be able to do anything significant with it for a while, so let them sit there for a while. I'd ask around, though.

Once you've solved the code page from the safe --

What do I do with the code page?

-- you can get into the red cards in the archives, and one of them tells you that those cards hold the formula for the virus. Yikes!

They also tell you that the cards need to be soaked in water (as did your dad, if you asked him about cards) and that they're no good without the ace, which is missing. Fooey. Well, let's soak the deck anyway while we go looking for the ace.

You need to do this in a private place, like Nancy's room --

-- but since she doesn't have a bathroom (what kind of hotel is this?!), just drop them into the vase of lilies on the table.

As for the ace, there's a clue to the location of the ace in the cylinder puzzle you found inside the fireplace safe at the cabin. But you won't actually get your hands on it till near the end of the game.

How do I open the cylinder puzzle inside the fireplace safe?

How do I find all the cards?