What is the most likely cause if you receive a parity error every time that you turn your computer on?

Your RAM is bad

RAM does occasionally misfire, and is capable of making mistakes. This is not typically a big deal for a home user, but it can be for applications that are mission critical in nature. Parity checking is responsible for adding more to the data used for error detection by the MCC. Parity RAM cannot correct errors, and may not always catch them.