Media Players not playing standard DVDs

[Rc mayhem]

My friend has a problem. She can't get DVDs to play on her laptop on either in the standard Dell player or Media Player. They used to work but now she gets the following message.

"Windows Media Player cannot play the DVD because a compatible DVD decoder is not installed on your computer."

She has some codecs such as divx to play some downloaded stuff. But I don't think she has updated these recently.

Apparently it doesn't now bring up the "what do you want to do with this DVD" message box like it used to.

Anyone know how to get DVDs to play again?