OK, picked up the Mega2560 from my friend last night, and tested it out on the Lenovo Win7 32bit desktop and HP Win7 64bit laptop, and it works fine on both. The only variation from the Uno is the driver takes a long time to load (2-3 minutes - OK maybe not that long in the big scheme) and opening the serial monitor takes a while to reset the Mega - maybe 5 seconds. The Mega works well on our LED and LCD shield and takes only slightly longer to load.
So the issue looks like a Dell issue. Maybe if anyone has a dead Mega2560, please report the make, model, operating system, and any other details which are show when you right click on My Computer and select Properties.