Wow, that board is almost 20 years old. And what OS will work well with just 1.5 GB of RAM, you'd be forced to use some frugal Linux on there... and even that wouldn't be good for much. You can get much faster bundles of board+CPU+RAM for very cheap, a lot of people are even giving more modern systems away for free or give them to electronics recycling. Often times, when I replace people's old system with something modern, they just give me their old hardware, and usually that's newer than this. So honestly, I wouldn't waste too much time on this old hardware, it's not worth it anymore, unless you want to build a retro system for old games.
Anyway, first you can try to update the BIOS to the latest version from
https://www.msi.com/Motherboard/945PL_Neo5/support
Try all three modules in the first slot, to make sure all modules are working individually. BTW, using three modules is no good, the third module will run in single-channel, because it doesn't have a counterpart in the fourth slot. But that's probably the least of your worries here.
Then, we have to consider that this board still uses electrolytic capacitors, not solid capacitors. And after 20 years, those capacitors might have lost enough of their liquid electrolyte that certain things don't work properly anymore. The same goes for the PSU by the way, which probably also would have to be checked, because if that is also close to 20 years old, it might have horrible output quality by now.
I marked some of the electrolytic capacitors on this picture, so you know what I'm talking about:
So those cylindrical things, those are all such capacitors, which are prone to bulging and leaking, or generally losing their electrolyte over the years.
Bulged capacitors.
If they leaked, you can find some odd goo on top or around them, and from the pressure, the top will have bulged out or even vented.
So you need to visually inspect the board. If you find any capacitors that look odd (anything other than a perfectly flat top), then this board needs to go to electronics recycling. Theoretically, it can be repaired by soldering new high-quality low-ESR capacitors on there. But you would almost have to replace each capacitor on there, because they will all be at the end of their life, so it's just not worth even spending ten bucks on this old board, let alone do all the work.
