Done unplugging all devices - no changes. tried to boot in Safe Mode with Command Prompt -no keyboard or mouse...
Here are some snippets from DELL website, please take your time to read it and apply as required:
Make sure that you are using the correct ports and also read extensively about theI/O-Panel Components on one of the links provided below.
A USB Device Is Not Detected After Reseating on a Dell™ Dimension™ E520
A USB Device is Not Detected
When a USB device on a Dimension E520 is reseated into the same USB port, or swapped between USB ports, the USB device may not be detected by the system.
USB Device Does Not Work When Connected to Desktop Computer
Journal ID: 10206J5M9F
Wizard ID: 110448
Date Published: 8/14/2003
Last Revised: 8/21/2009
STEP: Ensure That the USB Device Is Attached Directly To the Port
Additional Information
To ensure the device is attached directly to the port, verify that the USB cable runs directly from the peripheral to the USB port, with no intermediate devices in between.
How to Run the Dell™ Diagnostics Utility
Removing and Installing Parts
http://support.dell.com/support/edocs/systems/dimE520/en/SM_EN/parts.htm#wp1184332
Try to make sure the I/O-Panel Components fitted firmly
Extended Test Dell Diagnostics Main Menu
http://support.dell.com/support/edocs/systems/dimE520/en/SM_EN/adtshoot.htm#wp1059431
On the same page locate "System Messages " and the "Beep Codes " read what can be done and apply.
USB Controller :Set to On (default) so that USB devices will be detected and supported in the operating system.
http://support.dell.com/support/edocs/systems/dimE520/en/SM_EN/syssetup.htm#wp1055144
On the same Page above Under "Maintenance" see this option I mentioned in one of my earleir post:
Load Defaults :Allows you to reset all options in the setup menu to the factory default.
nass -- http://www.nasstec.co.uk