So I picked up Civ V at my local gamestop after work today.  When I got home I plopped it in my disc drive and got the installation started before heading into the bathroom for a shower.  When I got back I found that my Steam account was open and I was in the middle of downloading the 4.5 GB game.  I have the disc in the drive, so why am I downloading this on Steam right now?  What's the deal?

I'm not so sure about Civ 5 actually, but when i downloaded orange box(half life ep1+2) from the disc same thing happened to me. I tried to install it again and this time it got properly installed. So trying again might help you. Unless it's a HUUUUUUUGE patch that you're downloading...

It says that it's updating the game, but I figured that a 4.5 GB patch is really the entire game compressed from the required 8 GB of available HDD space.  I'll try reinstalling and see if it works out.
 
I really don't want to wait hours for the download to finish.  The reason I buy most games on disc is because my connection isn't that fast.  Thanks for the reply.

I have no idea why it would have to download so much from steam?
 
In the meantime, maybe you could disable the internet connection while the installer completes to save some time. Then steam should see the installation and only download patches.

" @Spoonman671: Steam achievements and DRM. "

Pretty sure Civ V requires Steam. 
 
 
Who knows what the 4.5 gigs that you're actually downloading are, though.

Okay, I think I figured it out.  Instead of choosing the reinstall option, I completely uninstalled.  Steam is getting the game data off the disc now instead of downloading.
 
I think I might spend the time I've saved downloading the currently on sale Left 4 Dead bundle (1 & 2 for $10!).
 
EDIT:  Oh, and thanks for the replies, guys.

Looks as though you've worked it out but I just want to thank you Spoonman671 for letting me know about the Steam sale on L4D 1 & 2...great deal.

I think it's just one of those Steam things... i bought Saints Row 2 on Direct 2 Drive and installed it... next thing i know Steam wanted to download the WHOLE thing again before i could play it.

The disc propably doesn't actually have the game on it. It's fancy drm. As far as I know, they went to great lengths to try and make Civ V difficult to crack.

"The disc propably doesn't actually have the game on it. It's fancy drm. As far as I know, they went to great lengths to try and make Civ V difficult to crack. "