Thanks to all for the help and advice.
Ran the Codec Sniper and loaded Codecs (even though I think I was up to date with everything I needed). No change to the locking up problem.
Running XP SP3 with all the patches and WMP 11.
'E-WMA-MP3-Converter' finds the .wma file and attempts to convert to mp3, creates an mp3 file name but then hangs after 2 seconds with the mp3 file having 0 Bytes in it. It's not that particular wma file that fails, they all do it.
Can only recover with Task Manager.
I suppose I could try Real Player to check the wma file will play with that, but I really don't want Real Player anywhere my PC!
Allergy Advice: This post may contain traces of Meerkats.