I installed Nero and it switched my default media player and picture viewer to nero products.
How do I switch them back?
To assign an application to each file type, simply right click on a file and select “open with”. Then select “choose program” and select what you like
Well each type of file you can right click on it “open file with” find the program you want it to run with. Check always open with click ok. done.
or open any folder under tools options file types and it will list every file assocation there. Find the types you want to change and click change, edit the open with. Just love programs that assume you want to use their stuff to work with something else without asking 
Don’t remember if nero askes during install, been a long time. Be glad you got the program to install anyways. 
Nero asks what it can take over when it installs, you might want to pay better attention to such things next time!