my power tab keeps say 'not enough memory to run this file, please exit other windows and try again'. and then it say 'unable to play MIDI stream file'.

Do you have a lot of programs running at the same time? It could be that all your memory is in use (including virtual memory). Try exiting some programs and seeing if that makes a difference.
Hm. Does it do it with every file or just one specific file? And is this a recurring problem or have you only just encountered it? I've never come across that problem with Power Tab before (though I have with other programs).