Please provide more detailed system information.
Is this system a Frankenstein (home-built) or purchased from manufacturer?
Windows 8 works but Windows 7 doesn't. What installation configuration are you using? Dual Boot, Virtual Machine? 2 partitions on the same drive or a separate hard drive?
When you installed Windows 7 and the system was working at first, did you install Chipset and Hardware drivers for Windows 7?
Try powering down the system, disconnect the power cord, keyboard and mouse, hold the power button in for 10 - 20 seconds. Reconnect the standard keyboard and mouse, power cable and boot the computer into Windows 7.
Does Windows 7 recognize the keyboard abd mouse? (it may take a minute or five to complete the recognition and device installation)
If Yes... Rock On! Make sure you install your device drivers for Windows 7.
If No... Shut the system down and reboot pressing <F8> repeatedly until the Startup options menu for Windows 7 appears. Select Safe Mode and press <Enter>
Does Windows 7 recognize the keyboard and mouse in safe mode? (it may take a minute or five to complete the recognition of the devices).
Best of luck and Please keep BC posted!