At the moment, if there is a problem with any of the devices, it's more likely to be the 5D, as Karen has already said she can transfer the files from the card if she uses the 7D to do the transfer .. just not with the 5D.
So the problem is either the 5D, or more likely a driver issue with Windows/USB chip in connecting to the 5D.
So far as I can understand tho, is that Karen has said that the 5D does successfully connect as a device in Windows, so at least it does connect .. now it's time to determine if the USB driver is causing the problem.
Connecting to a USB3 port will use a different driver/chipset for sure, and as we don't know which USB2 ports use which chipsets on the motherboard, the best way forward for now, to determine where the problem lies, is with connecting via the USB3 ports.
@ Karen:
When you connect the 5D to the PC, Windows should automagically produce a notification that the 5D has been connected.
The actual notification is a bit strange in the way it can work.
On my Win7 PC, it doesn't actually open a window directly, just a small notification in the task bar(I actually prefer this to an annoying window too).
If you click the icon in the task bar, it then opens a window which gives you various options for communication to the 5D.
Do you see this notification and pop up window, and if so have you tried connecting/transferring with these recommendations.
The options you may see are
could be differently set up for respective PCs tho)
Manage media on your device
Browse files
Import Pictures and Video(here you can also which program is the default for transferring your images)
Change general settings(here about the only useful option is to tick/untick the notification for when the device is connected)
Also in the window(which is basically a Windows Explorer window) it shows you the battery level and the total amount of storage on the device.
If you have two card slots and both have cards inserted, it will show the total amount of space for both cards combined. If you click the storage icon, it will then take you to another window where it shows each individual card's space availability.
Do you see any of this when you connect the 5D(and turn it on obviously).