Too, if you're melting it in a microwave, chances are that you're scorching it, which a microwave does extremely well. Once it has either seized (in the presence of water) or scorched, unfortunately you have to kiss it good bye and start over.

Yes, how are you melting it? When melting in a bain marie, be sure that you keep the heat to a simmer so that steam doesn't billow up out of the pot and into your bowl. ;0)

Sounds like your chocolate is seizing. This is due to poor temperature control. Have you tried using a double-boiler? This is done easily. Take a smallish pot, fill it with about an inch of water, bring to a simmer, not boil. Place a heat proof bowl on top, i.e. glass/metal. Place chocolate inside and whisk gently every 30 seconds or so until the chocolate is smooth.