I have one on the bench at the moment and it does exactly the same and I know the hard drives gone. When some machines boot up they check the drives and if they get no response they carry on. But if they get a partial response they get stuck.
Disconnect the hard drive and see if it will go into the bios.
Just an idea.
Any news on this did you solev any of the problems?
Being as they were donated PC's and there's some still to come i'm going to put them on the "needs fixing in the future" pile, i think it's the mobo i unplugged *everything* bar the power supply and took out the cmos battery and went from there plugging things in one by one and tried with a known working power supply.
In short, they're dead, however during stripping one down i noticed they had no thermal paste on the CPU atall haven't tried it with a known working CPU, might be worth a look