You know diagnosing a problem or helping in a 'how to' is far easier if the poster gives information about the hardware etc; The question is a bit lacking in detail, what internal mic ?
The more informative you are the easier it is to help.
My Computer
At a glance
Windows 7 home premium x64AMD FX-4100 AM3+ 3.6GHz 12MB Black EditionCrsair vengeance 12Gb DDR3 1600MHz CL9Asus GTX 560 1GB
Hello, I guess you're using Windows 7 =)
On the taskbar, near the clock, you can see an icon with a speaker emitting sound waves on it, right click on this, recording devices and then, find the microphone that is integrated, then disable it.
My Computer
At a glance
Windows 7 / UbuntuAMD 2.90GHz dual 5700+4.00 GBs of RAMATI HD 4850