Ok, so it was installed as a "Default Monitor" on my other workstation.
I downloaded that driverset and it installed fine and is recognized as a P991 monitor. Maybe you're not installing it properly?
Here's what I did:
Go to graphics properties.
Select the settings tab.
Select Advanced
Select Monitor in the new window that pops up.
Select Properties.
Select Driver
Select Update Driver
Don't let Windows contact Windows Update
Select "Install from a list or specific location"
Select "Don't search, I will choose the driver to install."
Hit the "Have Disk" button.
Point it to the folder where the file extracted the driverset, it should pickup that dellmon0.inf
P991 should be listed, select that.
And you should be done! Try that out.
You might also want to make sure your video card is installed properly too.