Just to check, does this happen with every CD you try to rip to .mp3?
I've just ripped a CD with WMP11 and had no issues regards to .mp3 format.
Let's look at a solution for you, first off try changing the bitrate to anything other than 128kbps, should do the trick without having to do anything more complex.
If not then try this solution:
Open the registry editor by typing regedit into the run dialog (Win+R), and off we'll go.
This one is recommended by Microsoft.
Browse over to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MediaPlayer\ Settings\MP3Encoding.
Double click LowrateSample, delete the value that's shown under Value data, and click okay.
2. If that doesn't work, try making sure that the PrefferredCodecPath key exists, and its value is C:\WINDOWS\system32\l3codecp.acm.
If you are not comfortable with editing your registry, which can be disastrous without backing up first, then you can try install and uninstall WMP, and if that still does not work, see if a downloaded
Codec does the job.
Try installing the Codecs first before uninstalling WMP11