Except for the keyboard & mouse, disconnect all devices from your computer. Try running +repair disk+ from your *software install DVD* and afterwards, run +repair permissions+ *from your HD.*
Your system info is incomplete. Which model G5 and OSX are you using? Please add this info along w/all the other requested info in "My Settings."----> Need this info so that the users here can provide you w/proper troubleshooting solutions.