That's not that uncommon really, and one of the basic troubleshooting steps of any boot related issue is to disconnect all non-essential devices from the computer.
If swapping the unit out for another one isn't an option, then you're probably better off explaining to the user she'll just have to plug in her flash drives and whatever else AFTER the computer finishes booting. If I had to guess as to why this is, I'd say that she probably has a number of USB devices, more than one are of suspect repair and/or origin and somewhere along the line that device (or devices) damaged the USB controller on the motherboard. Assuming my hypothesis is correct, it probably is in the very early stages of failure, and you should expect it to fail outright at pretty much any time, though it may well continue on as it is now for years.
You can also try reloading the OS, hoping you get lucky and it solves the issue, but I doubt it will.
I'm having a strange problem with a user at our company. When she boots, while a usb device (doesn't matter which one) it boots well, but after you get the screen 'Starting Windows' the screen gots disconnected and it is like when you put on the screen before booting the pc. The button of the screen is flashing.Anyone has an idea?