As you can see in the screenshots, my Sims and UI looks like I'm playing in an 800x600 window. I tried the Graphic Rules Maker, setting stuff up manually, and there are no changes. I'm confused, I reinstalled the game twice and this keeps happening. This has been happening since the first time I reinstalled the game, before that, everything was fine; the resolution worked properly. Third picture would be how my game used to look like.

You need the graphic rules and the 4 GB patch. Did you watch the video? http://modthesims.info/showthread.php?t=610641 Also this thread on Leefish should help http://www.leefish.nl/mybb/showthread.php?tid=5446
No changes meant you didn't apply it correctly is all.

Looking at your pictures it doesn't say that but I notice you have no refresh rate. What does your config-log.txt read in the Logs folder in The Sims 2 folder?

That does not look like 800x600 to me, since it fills your entire screen and also looks like widescreen (800x600 makes a tiny almost square window appear). The sims look fine, but if you mean anything is wrong, maybe give some details?. What is your screen setting? If it is close to 1920x1080 the resolution looks like it is where it's supposed to be.

The UI is small and doesn't fill the screen when you use a large setting than the default ones. Are you sure you didn't use a lower resolution when the UI looked bigger? It tends to shrink down a bit when the resolution goes higher than the original default settings.

Only thing I can think of (outside the Graphics Rules) is whether the "square pixels" settings may help. it has something to do with how the game shows up on a widescreen, but I'm not sure if that's mainly for a full-screen game.

I didn't know I applied wrongly my 4GB Patch... silly me. Thank you guys, the game is back to normal.