What will happen when I boot up off of an OEM Hard drive

[Jokerspades]

I am building a computer and I will be using a HDD that was from a prebuilt computer and I will be upgrading the os from windows 7 oem to windows 8.1 retail. When i go to start up the computer what will happen? will i see windows 7 starting up or will it say an error?

 

[quilciri]

A windows install on one motherboard will not work well with any other motherboard (except for the same make & model motherboard), NTM it will also deactivate the windows validation.

Since it appears you have a retail copy of windows 8, your best be is to do a clean install of 8 after you have moved the HDD to your new computer.