Open the Control Panel "Sound" option. Click on the "Recording" tab, and look for an entry for the microphone. Click on that, and then click on the "Properties" button. At the bottom of the window you will see a selection entry with the label "Device usage:". Click on that and select the "Use this device (enable)" option. Then click either the Apply or OK button. Your microphone should now work, though sometimes you may need to reboot the system.
Is your computer definitely a HP 7600 or could it be a D7600, even a DC7600? This would matter significantly when it comes to drivers.
Run a Windows Update, or check if Windows is waiting for you to give the go-ahead to install updates. Sometimes Windows fixes this issue "itself".
If it is that XP drivers ave the only drivers you can get, then you may get away with installing the HP driver installers with Compatibility set to 'XP Service Pack 3', and ticking the 'Run as Administrator' box.