Why does my DVD player software on computer change DVDS so that i can no longer watch them on my DVD player?

The most common reason this happens is due to buffer under-runs. CD writing, and DVD writing ( burning ) is a very delicate process which requires high amounts of system resources. Running other programs actively such as browsing internet, or having other idle programs in the taskbar running can greatly depreciate your resources, depending on your system. Your software reads ahead of the burn, and creates whats called a buffer. If the system becomes busy while burning, the data is used in the buffer. Similar to the operation of a skip-free CD player reading 5 seconds ahead so that there is continuous playing when it receives a bump. If the system is busy for too long, and the buffer runs out, this is called a buffer under run, and can damage the functionality of the CD/DVD.

Closing all programs, and not using your PC during the burn should help. Be advised, that depending on your systems specifications, even an intense screen saver could eat up memory that the burn process could be using.