the bottom line with the OEM Vista is that you are allowed to install it on one machine, as many times as you require.
however you are not allowed to ever install it again on a different computer, even if you have formatted the first computer.
If you upgrade your computer (i.e. change the CPU, RAM, Hard Drive, Graphics Card, Sound Card, PSU, Case, Any Optical Drives (DVDROM, DVDRW+-, CDRW, CDROM, DVDROM CDRW Combi as many times as you require) you do not break the user agreement.
But if you change the Motherboard it counts as a new computer, regardless of whether every single one of the other components are the same as they were on first install.
The ONLY time you may change your Motherboard is via the warranty, say if your Motherboard breaks and you get a new motherboard under the warranty. However that Motherboard has to be the exact same make and model as the one it is replacing. However if say the manufacturer has stopped producing your model of Motherboard and has to replace yours with another similar model (basically something the same price with similar spec) that would be ok, however Microsoft reserve the right to question any warranty replacement and ask for proof.
The Difference between the Retail license Vista is that you may install your Vista on any computer then format that computer and install it on another computer. you can basically install the Vista on any machine, However it CAN ONLY be on one machine at a time. For example say you have a PC and a laptop. You can install Vista Retail on your laptop just to try it out for a month or two, then format your laptop and put your Vista retail on your PC.
I hope this has cleared things up.
