I have Portal 2, it's the disc, for the Xbox 360. I was wondering if I could install the game on Steam using that disc. It's hard to play on the Xbox sometimes, and the computer controls are much better. I've tried multiple ways, such as doing the "C:\Program Files (x86)\Steam\Steam.exe" -install E:. There seems to be no other ways, I've looked everywhere, and... yeah. If anyone can help me, thanks in advance!
No, Xbox 360 discs cannot be installed on a PC. If you'd like to play Portal 2 on PC, you would need to buy a separate Steam copy.
The Xbox 360 is a different operating system from Windows, Mac, and Linux. You cannot install software intended for one OS onto a system with a different OS. They are too different. That is why different versions of games are released for multiple platforms (For instance, a game could be release on PC, Mac, Xbox 360, Xbox One, and PS4!)
Because of this, you are basically stuck rebuying the game on the proper system if you want to switch platforms.
Portal 2 is $19.99 on Steam and most other outlets right now.
Historically, Portal 2 has gone on sale very often, and has gone as low as $4.99. If I were you I would wait and monitor for sales.
isthereanydeal.com will help you monitor for price drops on games. The link I've supplied is for the Portal 2 section where you can hit "Wait for a better price" and define exactly what price you'd like to be notified at. You will receive an email when that price condition is satisfied. That way you can make sure you get Portal 2 for whatever you think is fair for a second copy.