Steam Install Question

This may be common sense to most of you but since I avoided Steam until OB was released I know little to nothing about it. Last year I bought a retail copy of Titan Quest and after playing it for a bit I bought a retail copy of the expansion Immortal Throne. Now that I have my new comp I want to install the game but can't find the original disc or box, but I still have the expansion. If I download the original game from Steam can I install the expansion using my retail disc? Assuming I can, after I install the expansion will I still have to run Steam to access the game?

 

Civilization IV expansion packs work the reverse way (Steam expansion to retail box original) but I gather not the other way around (buying the original on Steam and then trying to upgrade with a retail box expansion). I would search on some forums before you buy a Steam copy of the original.

Your solution for this game is to forget all that hassle and just get Titan Quest Gold on Steam for $20, the same as buying the original. It contains both the original and the expansion for the same exact price. Forget about your hard copies, when they're giving it away for free on Steam, assuming you're ready to sink another $20 on the game that you lost.

 

It should, in theory work. It's just gonna add onto the installed game... shouldn't matter what client installed it.

 

It should in theory, but I have heard that it doesn't in practice with Civilization IV (as an example). It sounds like you're spending the $20 anyway, so you might as well get the Gold pack, since it's the same cost.