Whenever there is a new version available of WMP, a window pops up asking for upgrade. To disable this, follow the steps given below.

• Click Start -> Run -> type regedit and press Enter
• Go to HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ MediaPlayer\ PlayerUpgrade
• Create a new string value with the key AskMeAgain and set the value to no. (if the key AskMeAgain already exists, then just modify it. Close your editor now.
• Please Log Off or Restart your PC for the changes to take effect.

Remember to backup your registry before trying the above trick because even a small mistake can do serious damage to your PC.