Depends -- Technically the license is tied to the original MOBO that the OEM version was installed on so if you are using the old MOBO then you should be able to use the OEM Vista -- In reality MS will normally help get you authenticated if you run into problems after installing an OEM version of the OS if you have the Product key (esp. if you call and explain to them that the old MOBO died so you had to replace it and other parts.) --- The EULA states that it is tied to the original MOBO but they are usually willing to help if you contact them as long as you are not using the same license for more than 1 system.
I recently built a new system using just the old HDD, VISTA OEM disk, DVD drive, Mouse, keyboard, Powersupply and replaced everything else MOBO, RAM, CPU, GPU, new 500gb. HDD which I installed a fresh install of VISTA using the OEM disk, etc. and all I had to do was use the Phone activation process to get it activated (you read off the product key and the computer operator gives you a response code to enter in the activation window) and it has been working fine -- so yes it can be done.