Windows mail

I have Windows Mail on a MS Vista computer.  Just started using the mail program.  At first I could read emails but now often get an error message: 

"Message could not be displayed".  Windows Mail encountered and unexpected problem while displaying the message.  Check your computer for low memory or low disk space and try again".

But if I go to another email program, I can display the same message.  Or, sometimes later I can display the message properly in Windows Mail.  Any ideas?

thanks,