Welcome to the forums and to DC ajnast4r!! :beer:
1. BOINC might be running all the time, but it doesn't do work if you set it to work only while idle. Just go to your taskmanager (right click on taskbar - taskmanager) and check under performance if BOINC is really doing work even when the PC is not idle. You can compare this to your car engine running while you're waiting at a traffic light. The engine is on, but waiting for you to speed up.
2. Yes, but only in a way that you're CPU might only last 8 instead of 8.5 years. Generally speaking, before you get to feel the extra wear on your computer, your computer will long have outlived its productive cycle. I've been doing DC on a PIII 600 for 4 years straight now, and it has never let me down, same for 4 other PCs and two notebooks. The only thing that died on me once was a case fan = 3 $.
If you have more questions, please ask!
:beer: