I had a client pc with Windows XP Pro installed brought to me with a problem of not being able to install some software. After looking at the pc I found that the DVD drives were not even listed in My Computer. The drives did have power as they did open and close correctly.

I checked the device manager and they were listed but had the dreaded exclamation points over the icons. After talking to the owner I was informed that they had tried to remove some software that was used for DVD creation and DVD movie playback. Apparently there are some know issues with uninstalling this type of sofware. Anyway I found a nice article on microsofts site that referenced this issues. Microsoft has created a Fix IT ( Microsoft Fix it 50027 to be more specific) that I ran and it replaced some registry entries automagically. After a reboot Windows XP detected that the drives were there and reinstalled the necessary drivers.
When the pc came back up I was able to now see the DVD drives listed in My Computer and was then able to install some newer better software using a CD.
It is worth noting that if the Fix It fails Microsoft also lists the steps manually to attempt to repair the problem on the article as well
This may not be the only way to fix this issue but IT FIXED it for Me.
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