Giant watch battery indeed... Actually used in Tamagotchis, too.
Ram is the easiest to test. You usually have at least two sticks. Simply take them off (there's a latch at the end of the slot) and try one at a time, preferably in several slots. Your motherboard manual should have info about where to attach an odd number of sticks, so check that out. If you can make your computer boot with or without one certain stick, you've found the culprit. But if your system boots with one stick but not with a pair or more, it might be the psu. Or motherboard... If you can make your system boot but can't access windows before rebooting, there's bootable cd/USB stick tests. More on the subject here:
https://www.howtogeek.com/260813/how-to-test-your-computers-ram-for-problems/Testing the gpu or cpu is not that easy. Basically the only way is to try with a known good one. And again, there's always the possibility that the symptoms are misleading and the true culprit is the psu or the motherboard. Anyway, if your motherboard has onboard video, simply take your video card off and try with the onboard one. If your system boots, it's video card related. Yes, unless it's because your psu can't feed your video card or your motheboard has a failing pci-e slot... But anyway, if the onboard video works, you then can try with another gaming level video card to see if you can boot with another power thirsty one. Vice versa, you can plug your video card into a known good system to see if it boots. Same goes with the cpu.
As you can see, it takes quite a lot of fiddling, swapping components from one system to another. That's why the easiest way is the first thing to recommend. So: Change the button cell. Take out the ram sticks and video card and clean the slots. Try with a single ram stick. Try with the onboard video if your motherboard features one. Check that the cpu cooler sits properly. Check that the cpu fan is connected to the right connector. If you have another fan, swap the connectors for case fan and cpu fan just in case the cpu fan speed indicator is broken.
If the above doesn't help and you end up swapping parts, notice that the test components need not be exactly similar to the original ones. You're not going to use them for heavy gaming, only for testing if you can make your system boot. Thus any underpowered psu would do as long as it has the connectors needed. Equally, an old video card would be fine if it has vaguely similar power connectors. Not every pin has to be used for testing. And of course same goes for the cpu, the weakest one of theright type for your motherboard will do just fine. Most likely you don't have all the testing parts at home, so ask your friends/colleagues/job or school IT dept/local shop for testing parts.
Good luck!
