You need to deal with the rot first, or it might continue underneath
the coating you apply.

Thanks, I hadn't thought of that. I also had a thought that the resin
probably wouldn't help if the wood was still damp (it is). I'll have
to dry it out first

Before doing anything, I suggest that you consider if you should try
to
"fix" the problem at all. Was the foundation and the deck really
designed
and built to support a room? I suspect you will have continued
problems and
you may someday face a building inspector that tells you to tear it
down.
Was a permit give for the original project?

I would tend to believe the best thing is to tear it all down to
the
ground and build new from the foundation up. Doing it right means doing
it
once.

Believe me, I've thought about it, and I agree that it would be the
best thing. Money and time are the two things restraining me from
doing exactly that (mainly money). I plan next summer to put a block
foundation under the portion of the house supported by the deck
(currently just posts and joists like most decks). As for the building
inspector, well I don't know if there ever was a permit, though I doubt
it. Since I don't have the money now to handle a job like that, I'll
have to take my chances with any future problems. I don't normally do
half a job, but I can't see any way to afford a job of that magnitude
right now. In the future, it might be possible. I may have to set
aside a savings account for just that sort of problem.

Thanks for your answers so far.