Computer Hope

Hi everyone,
For the past half year or so I've been dealing with a strange occurrence while playing some of my games on the computer. First of all let me just say that my computer is 10+ years old, so I don't even expect it to properly run games that came out after 2010-2011.
My problem is that some of my games use 100% CPU and they start to lag as soon as I try to open them. This has honestly perplexed me because this occurred with games that previously ran perfectly fine. I'm talking Harry Potter 1 and 2, Diablo II, Prince of Persia trilogy, all pretty old games and yet somehow my PC decided that suddenly it can't take it anymore, like it did for 10 years or so. It takes usually a minute to load the game and then another minute to be able to exit it because the computer slows down so much. This I found interesting because at the same time games like LoTR Conquest, KoAR and Skyrim, newer games run pretty well, no complaints there. So I wondered whether the problem could be related to games that require a disc to play but Diablo II (pirated version with disc in daemon tools) ran flawlessly a couple days ago and now that I bought the game digitally (so no disc) and try to open it the lags start coming like with the other games. Two Prince games are also digital but the problem persists. Another fact that might be of interest is that with some (not all) of the above mentioned lagging games I had to double click twice on their icons for them to start, which in the past wasn't much of a concern, they ran smoothly, but now it starts loading on the first double click but won't ever start up the game unless I double click again, but I don't think it's about running the same game multiple times simultaneously because the task manager shows that only one is running.
I've recently cleaned my hardware, got rid of viruses, there is plenty of space on all drivers, I don't even have internet connection most of the time. Sure, CPU runs pretty high even with playable games (around 95-100%) but that hasn't really caused any significant performance issues so far.
My specs:
Intel Pentium 4, CPU 3.00 GHz
RAM 3,00 GB
32-bit Windows 7 OS
GeForce GTX 1050 Ti
Besides the graphics card and CPU cooler no changes have been made to the hardware in the last 10 years. I know my computer is done for. I'm planning on drastically upgrading it in the near future but for now I just want to be able to play my old games.
Any help is much appreciated.

Video card is very bottle necked by the CPU, but I am thinking maybe you have a thermal issue with the CPU that is causing it to thermal throttle. I'd make sure the CPU temps are staying below 70C you can use speedfan to monitor temps of CPU/GPU and more. Speedfan is free. Additionally there is a tool out there that can detect thermal throttling however thermal throttling can also be seen with task manager and look at the CPU utilization and see if its solid 100% use or if its cyclic in CPU backing off of 100% as it tries to cool itself as its running. Other way to detect thermal throttling in some that will hold steady at 100% use is watching the clock speed of the CPU as it will speedstep to a slower CPU clock and that brings on lag as the CPU attempts to stay cooler by running slower yet 100% use.

A Core 2 Duo or better computer can be purchased these days cheaply as refurb or used. The Pentium 4 while your playing games of the period you may want to add more games to what can be supported and in that process you will want a true dual core or better CPU like a Pentium E5400 or Core 2 Duo or better. However cost of used computers have come down so far that for example I got a 4th generation Core i5 tower with 8GB RAM and Windows 10 64-bit for $109 on amazon a year ago with a warranty and the processing power of a 4th gen Core i5 is many many times more powerful than even the Extreme Editions of the Pentium 4 series as well as could save you money on your electric bill as well as so much more processing power for less power consumed.