Hard Drive addition

[Aaron]

I have an HP Slimline a little over a year old, out of warranty. HD
would not boot so took it to a Tech who said the HD was probably OK
but it has a serious hardware problem, and that it would probably be
cheaper to replace the PC rather than try to fix it. I did check out
the power supply per suggestions from this forum. That being the case
I bought a new HP Slimline. I then bought a HD enclosure, pulled out
the HD & installed it. Both drives have Vista Home Premium. Hopefully
the one out of the old PC still has all my info on it so I can
retrieve it. I have never done this before so I really need some
guidance.I will list some questions, & hopefully some of you experts
can help me. This is assuming that the old HD is in fact good.

1. Will the two drives be just like having two complete operating
systems operating through the same MB? Can I just use one or the
other, & if so how do I do that.

2. The drive enclosure came with software, do I need to install that
on the new PC, or given the fact that the HD has Vista on it will
that do.

3. Is it a good idea to have two drives with operating systems?

4. How do I swap files from one to the other?

5. I have not plugged the old one in yet to find out if it even
works. Is there anything that could harm the new PC by just plugging
it in to see if it will boot?

6. What about the old DVD drive, can I use it in an exterior
enclosure to copy from CD or DVD to the same in the other burner?

7. Is there somewhere online I could read about what I want to do?

I will probably have a lot more questions, but will start here. Any
advice on how to start, & proceed will be appreciated.