I am using Win7 x64 on an Acer laptop and have had the annoying loss of microphone volume a lot of people have reported with Win 7.

This fixed it for me:

Go to Control Panel and click on System, then Device Manager.
Click on Audio, Video and Gaming Controllers

I had NVDIA High Definition Audio listed 4 times (no idea why), plus a Realtek device.

I opened the Realtek device and clicked on "Driver" and then clicked on Previous version (might not say that exactly, I am not using English Windows). The previous version of the driver was installed and after a reboot everything works fine. I can now use the microphone normally (works with Skype now).

My list of audio drivers now shows the 4 Nvidia, plus "High Definition Audio device" (the Realtek label has disappeared)

Hope this helps.