Title :P I prefer installing expansions after playing through vanilla game.

The short answer is you don't if you have a version that comes packaged with the expansion. But you don't have to play the expansion areas if you don't wish to, although certain expansion added items will still be available, like scrolls etc.

Pfft, well, that is kinda problematic since my CD version doesn't work some reason after being installed :P

What do you mean by that? Is the CD damaged? Does the game itself not run? Does it crash? More details would help.

Well, after I have installed game and selected from launcher to play the game, a window pops up and says I can't run it :P

Well that's not much help. Where (which file path) do you have the game installed?

E:?

Which path, not which drive!

Be sure it's not in a protected space. You may need to turn on compatibility mode.

Most folks don't have much trouble with the CD version, though.

I had a problem with the CD-version of Baldur's Gate 1 as well. Got the error message saying that it will only run on English operating systems.

If that is the problem, you could try this: http://www.gog.com/forum/baldurs_gate_series/playing_solo_through_bg/post17

What path? <_< What does it matter?

It might be that it is, as Tallima indicated, in a protected space, which might interfere with the game's functionality.

Why so evasive?

Err, sorry, unclear grammar, I meant "What is a path in this case?" xP

It's the location of the game in your computer, as represented by a string of text representing a nested set of folders. For instance, the path I have for BG is "C:\GOG Games\Baldur's Gate"