Recently got a new PC and it displays the game widescreen, which I do not want. Any ways to change it?

You can use SADX Mod Manager to change window size, fullscreen and so on. :

Download link: http://mm.reimuhakurei.net/sadxmods/SADXModLoader.7z

You can also change that with the Autorun.exe on the Disc, but it's far less convenient.