Hello,
Are you relying on the on-board audio or do you have also a discrete audio card? Did you try also the front mic input?
Please check in Control Panel --> Sound --> Recording --> you should see at least three devices (2 x Microphone and 1 x Line In). Did the first / second device activates upon inserting the microphone jack in the front jack / rear jack? If activates, right click on it, go to Levels tab and augment the MIC volume and the MIC gain (the settings are: 0, +10dB, +20 dB, +30 dB). Also, you should check if on the Mixer menu in the right lower corner of your taskbar appears the Microphone tab when inserting the plug. If not, you should reinstall your audio card by using the latest drivers from manufacturer's web page.
Have you tried with some cheap mic, such as the gaming headphones do have? If such microphone works, maybe your audio card doesn't work well with dynamic microphones.
For better recording quality from microphones, you should consider a discrete audio card, such as an Creative Audigy or X-Fi, but only if the onboard audio quality is not sufficient for your needs.