I haven't used the D'Accord stuff, but given that they seem to be a
commercial organisation, I suspect they've developed stuff that either
uses sampled sounds or a decent synthesised implementation.
Powertab however is free software that outputs MIDI information to a
MIDI device. If the only MIDI device you've got on your system is the
Microsoft Software Synthesiser then the sound is going to be pretty poor.
You could take a look at:
http://www.power-tab.net/getarticle.php?id=91
You could presumably download some standalone software synths from
somewhere that will accept MIDI input , then use something like LoopBe1
(http://nerds.de/en/loopbe1.html) to connect the output of Powertab to
the input of the soft synth.