Hi, I'm running a Windows 2008 32 bit server as my domain controller and file server and web server and thought I could run a bitcoin miner on it as it is running anyway 24 hours a day. The only problem is that it is running on very old hardware (around 5-10 years old), so no GPU support and even the CPU is slow (Pentium 4, 2.8GHz). But as it doesn't cost my anything to let some software run permanently on it, should I do it or is it not worth it? I would also have an unused latest generation (around 6 month old) Mac Mini, but I would have to turn it on which costs energy (0.1787 CHF/kWh at day, 0.1167 CHF/kWh at night). Also I have several PCs available that could get the software installed, so they mine in the background, but none has strong graphical power.

Questions:
1. When I run the official Windows client, is it mining? Or is this client only for transactions?
2. Why is the official client using exactly 50% CPU? If it's mining, then I would expect 100% CPU, if not I'd expect ~0%.
3. What software should I install on my old server to do CPU mining?
4. What are the security risks running a miner on a server having the ports open? Or can mining be done "offline", transferring the results only manually once a day or so?
5. Is it worth powering up the Mac Mini running any miner? If yes, which one?
6. Is it worth letting *any* mining software run on the server or is this way to slow anyway?
7. The official client didn't ask me any setup questions. Where are my bitcoins if I get any through mining or transactions?
8. How would I join a mining group? Do I need special software for that? The official client has no options.

Please no guessing. But if you have a clear answer to any single question, I'd greatly appreciate the answer. Or ask if you need to know more details. Thanks for helping.