ptracton:

You might be experiencing an issue with the system's bios or settings. If it is a Small Form Factor GX620, it is not unusual. You might try swapping the ports that you connect the peripherals to. Try using ports in different groupings even using one of the front port locations. You may need to enter the Bios setup to ensure that legacy support for USB devices is selected if it is available. To enter the Bios setup routine, press F2 or Ctrl+Alt+Enter repeatedly just after the machine is powered on and just after the keyboard lights turn on and off. The machine should then enter the Bios setup. Navigate through the menus and look for USB settings such as legacy support. Enable the setting and exit the bios while saving the changes. A Bios update might also be available to address the issue. Look at the Dell Support site, enter the tag number of the system and search through the updates or download section for Bios updates. Look at the notes pertaining to the update to see if it applies. Be careful about doing a bios update. It can cause more issues if it is done improperly or could render the system inoperable.

There is also a routine that Dell has for clearing the NVRam and defaulting to the original factory settings. Might work on your system, might not. Look here if the link works: http://support.dell.com/support/topi...08F95605D14C94
If the link doesn't work, let me know and I'll PM the details to you.

Other than that, I can't think of anything else offhand, except trying a different mouse or keyboard, or throwing it out and getting something more Linux Friendly.