Here are some steps to troubleshoot the issue.
1. Verify that the PS/2 keyboard is plugged into the purple port and mouse is plugged into the green port.
2. Remove the graphics card and reseat it.
3. Remove the graphics card card, clear CMOS, and then reseat the card.
4. Try the graphics card in another system to see if it boots.
5. Try booting with a barebone configuration with the motherboard outside of the case. i.e. on a non-conductive surface like a cardboard box or rubber mat with only CPU, heatsink, 1 stick of RAM, and 1 graphics card.
If possible swap out the card with a known working card. If the issue persists try the card in another slot if applicable. If the system boots either fully, or asks you to move the card to the primary slot, you have a defective slot. If the system remains with error 25, try removing any PS/2 devices all together. This error is generally linked to P/S2 devices in the wrong ports, a defective PS/2 port or device, a defective graphics card, or a bad video card slot.