AMD legacy pc will on boot on second try

Have a user with AMD duron pc on an abit MB. has a network card, video card, cd rom drive and hard drive, nothing unusal inside at all, and I exchanged the ps to a 400 watt ps.

Problem is that the machine never boots on the first attempt, but will always boot on the second attempt. Can anyone tell me why, and if I can fix this problem. It’s not a major issue, the machine is stable on the second boot, but I’d like to resolve this for my peace of mind.

thanks