Check the obvious corrections first:

1. Open your Volume Control and check to see if the microphone is muted. For access: rightclick the volume icon on your taskbar and select open volume controls. When the "Play Center" controls appear, go to options, then properties, then recording, and make certain that microphone is checked. Then return to the "Play Center" controls (where you will now see a controller for microphone) and adjust the fader to an appropriate position, as well as uncheck the mute button if necessary.
   
2. If you have a SoundBlaster soundcard, you can double-check this procedure by opening your SurroundSound Mixer deck. The microphone controll (if not visible) is found by selecting any one of the icons above the columns, then by selecting "microphone" from the context menu. When the microphone controller appears make the appropriate adjustments. If there is a + above the column, click it open and make certain that the box is checked for "Mic Boost".
   
3. Make certain that your microphone is connected to the Microphone In jack (or red jack) on the back of your machine.

If you need to post back, please supply your operating system and soundcard and model Dimension for any of your readers who do not posess the ability to read your mind. Here is a link to an extremely helpful tutorial on Delltalk's hidden secrets.

daShadow, your point 2 below, click the red plus mark, has saved my Mic.  Thank you very much.  I have been reading a lot of Mic posts and I was getting worried, and then I came across your post.  I opened my Sound Mixer and saw the red plus sign and clicked it.  I checked the box and now my Mic is working great.  I do a lot of sound recording and editing and your suggestion has really paid off.  I really do appreciate it :).