You can automatically adjust your microphone volume in Windows XP using the Microphone Wizard.  This utility located in:

Classic View:

Start>Control Panel>Speech

Category View:

Start>Control Panel>Sounds , Speech, and Audio Devices>Speech

This opens the Speech Properties Window:

Next, click the "Configure Microphone..." button.  The Microphone Wizard window appears next:

Next follow the instructions to tune your microphone level to your voice: