We need more information to help you.
What operating system are you running?
What exactly did you delete when you said you "think" you deleted the software"?
Are you saying you deleted the sound card device driver software using the Windows device manager or did you remove some other audio application?
I will try and help anyway with minimal information so far.
If you're running windows XP (vista is similar) I would check if the driver software for the sound card is still intact. The microphone itself has no "software" exactly. It's just connected to the Internal sound card and the sound card has a driver program that allows it to communicate with the operating system, and thus it can provide recording or other microphone functions.
1. Check if the sound card device driver is still there.
Right click on "My Computer", Select "Manage" and go to the "device manager". Look for an entry in the section most likely called "Sound , video and game controllers". If the drive software is missing there will be an entry there listing a sound device with a yellow exclamation mark next to it. It may also end up in some other generic section of that same page since Windows may not know how to categorize the device if the driver is missing.
If the sound card is OK it will be listed there and if you look at the details it would say no problem is detected.
2. Make sure the laptop speakers are not muted or turned down.
3. Make sure the mic itself is not muted also or turned down.
You can view those controls by double-clicking the speaker icon down near the clock in the lower right corner, or by going to in the control panel > sound-audio devices.
4. Check the mixer levels for both play and record, you can only view one at a time.
That means there is a mixer for playback and another one for recording. If your device driver is missing you have to fix that first.
Even if you remove the "driver" the files are still on the computer unless you proactively went and found the driver files (not likely) and removed them. For example, if you go to the control panel and delete the sound card entry, it will come back when you reboot because the driver files are still on the system.
You could go get the latest sound card driver from Dell.com.
Go to the Dell home computers > Support page, Drivers, Laptop > Lattitude, etc and put in your model number of your laptop or the service tag number from the bottom of the laptop. I won't cover the exact sequence since you should be able to find it in about 2 minutes.
This direct link might work (take out spaces if they show up)http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=19&l=en&s=dhs&ServiceTag=&SystemID=LAT_PNT_PM_D600&os=WW1&osl=en&catid=&impid=
Dell drivers are a single file that is self extracting after you download it, and then go find it and run it. It will install the appropriate driver files when you double-click on the installer.
It appears the Dell D600 has a Sigmatel sound card by default..
We can provide a shorter and more accurate solution if you provide more details as indicated earlier.