Fix: Thanks Zexion; you pointed me in the right direction. When I had plugged in my laptop earlier, it turned off 'Extend Desktop onto Second Monitor'. For some reason, on my main monitor (probably because it's super widescreen), it scales the full screen differently if my desktop isn't extended.
I literally made no changes in my scripts or anything, but today when I ALT+ENTER to enter fullscreen mode, it no longer stretches across my monitor to be true fullscreen.
It's much more vertically stretched... I have no idea why RPGMaker XP is behaving differently with me doing nothing.
Did you change your computer's resolution?
What about the DPI setting? (Should be at 100% not any higher)
Check: On Windows 7/8 go to Control Panel > Appearence and Personalization > Display
Sometimes it gets changed by other programs or even on accident.
Also if none of these work thennnnn D: sorry? lol Idk