Welcome to CDF
Dodgy PSU/overloaded USBs? Have you tried the camera/pen drive connected to another PC to rule out a problem with those? Unlikely, but worth ruling out.
Do you have anything else connected via USB (printer, mouse, keyboard etc - with my two Dells, both keyboard and mouse are USB).
Reason I suggest PSU as a possible culprit is that, though I personally like Dell machines, their PSU's leave a lot to be desired