Actaully, despite what MS say, if you have an OEM version of windows it is locked to that system's brand of motherboard, not the model. It does this by verifying the manufacturer name in the BIOS. MS would prefer you not to install it on a different motherboard as you have "created a new computer" and would prefer that you purchase a new copy of windows for that "new computer", but they cannot and do not enforce this.
Personally I have successfully installed the OEM windows 7 that came with an HP Desktop on an old HP Laptop, and the OEM Windows Vista that came with a Dell Laptop on a Dell Desktop with no problems, and they have very different motherboards.
There are a large number of OEM versions of windows, for example HP, Dell, Sony. There are also various Motherboard manufacturer OEM versions eg Asus, GIGABYTE etc.
So for example, if you have the DELL OEM version of windows 7, you can install it on any DELL machine, not just the one that it came with.
If you have the Asus OEM version you can install it on any Asus motherboard.
There is no limit to the number of times you can reinstall or the number of different machines you can install on (provided they match your OEM version). However, when you reinstall you have to "Activate" Windows again. If MS find you with more than one copy of windows running and "Activated" with the same serial at the same time then they may disable your serial and you won't be able to get updates.
I have been running 2 machines on the same activated OEM serial and been able to get updates for several months without any problems, but there's a risk in doing this that MS will detect me abusing my license and disable it, so I cannot really recommend it...
You can defeat the BIOS brand check with a reasonably simple hack, and the Activation checks are defeated quite easily with the DAZ loader, but doing either of these means your system is theoretically not supported by MS, warranty voided etc.