PC Startup: 3 Short Beeps Error!

Hello,

a friend of mine wants to pass on an old PC (Pentium 4) to his brother and asked me if I could reinstall Windows on it.

So far, nothing more straightforward! I boot up the PC for the first time and recover a few files to back up. Then, I restart and there it is, the failure, the system freezes on the motherboard's welcome screen! Furthermore, I hear a short beep indicating an error! I check the meaning: memory refresh error! I remove and reinsert my RAM sticks. There, it works, the PC boots up normally but this time, I hear 3 short beeps indicating an error (which doesn’t stop Windows from launching). Meaning: memory test read/write error.

I try inserting my RAM sticks one by one but the result is always the same: 3 short beeps!

In your opinion, where could the problem come from? How can I fix it?

In the control panel > System, it indicates that I have 2.50 Go of RAM! Which I think is correct concerning the sticks I have (I have two of 250 and two whose capacity isn’t indicated but I suspect they must be two 750 sticks... is that possible?)

I also tried to access the BIOS but no luck! I press Del (as indicated for the setup) but nothing! I try F1 and F2, but nothing either!

Is it possible that it relates to the motherboard battery?

Thank you for your help! (-;