@Averex

I had no problems downloading the UC from Origin again a while ago - I upgraded my pc.

Repair UC as follows:
You right click on your mouse while in Origin (UC) and you then get an option: Repair game. Click on it. Start the game while you are still signed in. It sometimes take a little time (especially if the game has to regenerate cache files) - but after it has opened you should be able to close it, exit Origin and play without being signed in.

Sometimes, repairing the game is necessary after deleting the cache files - I do not know exactly why, but it works for me.
After adding new custom content, it is good practice to delete the cache files.

It seems to me that you have taken a lot of steps - if the above does not help, you may have to look at other problems like screencard that is broken, power supply not working properly, fans, etc. (These things will affect your game before it affects anything else on your pc since the game uses a lot of resources).

UPDATE: Somehow my game files got fucked-up, so I was forced to reinstall the UC yet again. I got the game working, but the Bodyshop that came with it still refuses to open. Just the same stupid "Please make sure that the Sims 2 or any of its tools are not running before starting this application" dialog box. I'm honestly out of ideas at this point...I just want to get back to making sims already!

@Justpetro I tried that, and it didn't do anything :/

@Averex - it is a tough one. All I can think of is that there must still be something of your originals Sims 2 installation somewhere on your pc. I certainly don't have any problems with opening Body shop with UC myself.

Have you checked your hardware?

@Justpetro Idk. I tried clearing my registry, and it seems that all traces of the disc version are gone. I can't think of anything I would have missed.

@Averex - what do you use exactly, since you can only have either the disc version of the game OR UC - you cannot have both and if you have UC you need to uninstall the disc version of Sims 2. All of it,

Registry Cleaners delete some files connected to the Ultimate Collection and that can be repaired in Origin and has nothing to do with the disc version in my experience.

I am not sure but you might have deleted something else with that registry cleaner too like part of a driver or something.

In general - since I have completely stopped using any registry cleaners and just the defrag and disk cleanup from Windows, I have had absolutely no problems getting my game or anything else to run.

As a last resort, I would suggest that you completely uninstall everything and then just download UC again. If that does not work you might have to ask a computer expert. Check your hardware as well - fans, power supply, the small things that the device manager does not always pick up (or ask somebody who knows computers to do it for you).

I know this isn't helping in any way, but I also forgot to remove my old disc install when I installed the Ultimate Collection. Having said that, I don't see any problems with Bodyshop when I try using it. So unless I just got really lucky I doubt failing to remove your old installation is the problem. All I did was remove the old Sims 2 files in the "Program Files" directory.

When I contacted EA about getting the UC I asked if I needed to remove my old Sims 2 installation and they said I didn't. Either that person was telling the truth, or didn't know what they were talking about. Either way I haven't seen any major problems with my game, and like I said I never uninstalled my old disc install.

@haywud - you are somehow, I guess, lucky - it's a game using plenty of computer resources, running two versions can only lead to complications.

Well just about everything seems to be pointing to the Ultimate Collection folder, and the only traces of the old install left are in the registry. So unless I'm missing something I'm not worried about the old install. At some point I may do a fresh install of the UC and remove those old registry entries first, though I will probably only do that if I start to see problems.

@Justpetro I tried that and it didn't work. Idk if the problem could be so severe that it actually goes down to the hardware, but maybe it is. I'm hoping it isn't though, since I highly doubt my parents would be willing to take my computer into a store to have it looked at over something like this.

Hardware is not always severe. Sometimes it can be a small thing like the power supply not working properly - maybe you should find a friend who knows a little about hardware to have a look first.

*decides this case is hopeless*
*crawls into bed and hides under mound of covers*