I'm thinking that I might change my CPU and my motherboard, but I would rather not pay an extra $100, so I want to know if I change my cpu and mobo can I reload Vista or do I need a new OS if I change the parts?
Yes, any time you add or change out the hardware, you always have to install Windows again. It's a pain, I know, but the driver conflicts would prevent your existing install from working properly.
It's a gamble - if it's an OEM license, it's linked to the hardware. Sometimes Microsoft will help you out if you contact them to deactivate/activate the license but it's a 50/50 shot. If it's a retail box copy, you will probably be ok. You could probably recycle the key if you purchase an upgrade to win8 if you don't have success with re-using your original license.
If it were me, I would just budget in a new license of Win7.
Yes, any time you add or change out the hardware, you always have to install Windows again. It's a pain, I know, but the driver conflicts would prevent your existing install from working properly.