E521 and WIndows 10

I had a couple of PC’s (Dell E521) left over from a business. They worked fine with XP, I upgraded the CPU’s to ADA6000IAA6CZ and maxed the RAM. They then worked perfectly with Windows 7, but with 10 neither will boot up with the first try on cold boot. Once they’re running there are no probs. BIOS is set to boot from HDD. I get messages in event viewer which tell me nothing.

Why, if this PC clearly meets the minimum specifications given as necessary by MS for Windows 10 to operate, do I have this problem, actually, just an annoyance? No crashes, no BSOD, nothing.

Thank you.