My computer wont let me watch movies now. it keeps saying something about my dvd decoder wasnt installed.

This is because you have not download a piece of DVD player program on your PC.
You can tryLeawo Blu-ray Playerwhich is a popular DVD and Blu-ray player program.
You can find more DVD player programs here:
Best Free DVD Player Software

If you bought the computer new or the DVD drive new , you whould have got a CD with it , most probably with something like WinDVD or something , if you have the specific CD with DVD software on it install it , as it will enable you to watch DVD's using Windows media player etc..

if you bought the computer second hand etc.. and or cant find the cd's a free solution is to download VLC media player (Download here) the only disadvantage about using this free solution is that you will have to play the DVD's through VLC you wont be able to use windows media player etc. to play DVD's as VLC does not require decoder software to play DVD's

otherwise you can purchase software such as WinDVD and this will enable you to play using your media player of choice.

Thanks!