Microphone is not working

Hi,
I have a compaq presario desktop computer. My microphone is not working
thou i am sure i have connected it properly. In settings I have sound
playback "Realtek Audio rear output" but i don't have any options
"Realtek Audio rear input" doesn't appear in sound recording but
"Realtek Audio digital input" does. These settings are gfrom control
panel. When i select "digital input" option for recording sound it
gives error that audio hardware can't record files into current file
and i have to create a new document. I don't understand this error
though.
Please help me out so i can get my microphone working.
Thanks in advance
Salil Jain