What version of WMP do you have installed? Have you tried the latest click here? If still no joy, try the free VLC media player click here. Actually on second thoughts, you may have to install software that originally came with the system in order to play DVDs - assuming playing DVDs is the only problem. Or, failing which, purchasing a DVD decoder click here. You'll probably get an older version of PowerDVD for a lot less on eBay click here.
Regarding the lack of sound, are there any problems in Device Manager? This will show up as yellow exclamation marks in most cases. Did you install the sound drivers?
It's also recommended to carry out a complete Windows Update (WU) click here session. Reboot the PC whenever it asks you to, & keep returning to the WU site until you've done them all. G