microphones

Hi all,
I've got a problem with using the microphones with windows vista.
I buy a new microphone, it works fine, and vista ask me to allow the use
(and I say yes) and after some days, from night to day, it stop working: the
laptop does not recognise it anymore.
At the beginning I though it was my stupid cheap microphone, but after the
same happened with 3 different ones, I thing there are some settings I need
to change.

Anybody got the same problem??
What to do?? (can't buy every few days a new microphone!!!)

Thanks