It's hard to say what the problem is here - are they the same M4A files you are testing? PlayIt Live is certainly unable to play M4A files which have Apple digital rights encryption attached.
I would suggest if you continue to have problems to convert the M4A files to MP3 or WAV which are guaranteed to play.
It is a mystery. Yes it is the same files, very odd. On Playit Live on my Vista machine they work fine. They will also play in VLC on the XP machine, but not in Playit !
Thanks for the prompt reply,