In Windows XP, we can enable Microphone Boost.
Go Control Panel -> Sounds and Audio Devices -> Audio tab -> [Volume] in Sound recording

Then, in Recording Control -> [Advance] -> tick [x]Microphone Boost

Enable this option will help the computer to boost up the voice. People will listen clearer.
In some of the Voice Chatting software such as Skype and VoipCheap, inside Tools -> Options -> Audio Settings.
There is a setting called "Let Skype adjust my audio settings". The feature will most likely disable the Microphone Boost which you have just set. So untick "Let Skype adjust my audio settings".