Can’t post with USB device

System has been running fine for about a year and a half with a PS2 keyboard and Mouse no USB devices on the system. Upgraded to a USB keyboard and mouse and the system will not post. I’m using a MSI K8N SLI 7100 motherboard with a AMD 4400+ processor. When the system stops the Dbracket lights are at processor init. If I let the system boot with the PS2 keyboard then plug in the USB devices everthing runs fine. I also get a hammer family processor unknown. I have cleared the CMOS updated the BIOS and all system drivers to no avail. I’ve tried several other USB devices like hard drives etc.. if there is anything on the USB bus includinging cell phones the system stops while the processor is triying to init.. Other than BIOS what could be happening here.