oh boy, you just opened a HUGE can of worms mate
right, heres how it is, and listen up before some other people get crazy and start spouting incorrect information
if you have the retail version with a disk, then you can install/un-install until armageddon, with absolutely no strings. you dont need internet, you dont need a disk, and im 99% sure you dont need a serial, but i installed my version (retail) a long time ago so i dont quite remember.
HOWEVER! installing from the disk does not give you all the upgrades. i think it will only give you v1.05. Right now with internet updates, original recepie Sins is on version v1.16 and Entrenchment (ill come back to) is v1.02. there are also new minor updates coming out soon, but meh
SO! if you want the most updated version of the game, or you want to play entrenchment, you need to get on the internet, download IE7 or later if you dont already have it, download Impulse, then buy Entrenchment, and update both games to the latest version (though you can upgrade your original Sins without entrenchment, entrenchment is just a micro expansion that adds some (REALLY GOOD) aspects to the game, your choice)
To download updates, you need to get an account with impulse, that means you need to give your email address and serial code and get an account, then register your game(s) which, if you bought them legally, wont be a problem 
SO, back to the beginning, if you are happy playing SP on whatever version the retail ships as, and dont want entrenchment, then you are sweet to go, absolutely no DRM, but, if you want to install updates, then you have to jump through a few hoops, but honestly, its nothing too hard or bad or anything, its just Ironclad/Stardocks preferred method of updating the game.
MOST IMPORTANTLY! if you are playing SP, YOU DO NOT NEED IMPULSE RUNNING, OR TO BE CONNECTED TO THE INTERNET TO PLAY THE GAME!!!!! AT ALL!!!! NADA!!! ZIP!!!! if you are playing MP then yeah, you are going to need the internet duh, but you still dont need Impulse running
alot of people cant get that through their thick skulls. the extent of Sins DRM is that you need to log in to Impulse to get updates. and even then, if they dont release updates (like right now they are busy on other things), you dont need to open Impulse at all! and you can play to your hearts content
but, the most important thing to remember is this: to update, you need to run Impulse with the internet. for MP, you just need the internet, no Impulse, for Single player, you can play in siberia by your lonesome without so much as a link to a power socket (laptop) and you will still be able to play
hope i cleared all that up for you.