Supposedly Windows 7 does not contain support for USB 3, but (again, supposedly) if you install drivers from the manufacturer, it will gain that capability.
I have a page with some recommended downloadable Dell-issed drivers for the Dimension 9100 model... but, it was originally released for Windows XP (which definitely did NOT support USB 3) ... so, maybe the drivers will help, maybe they won't, and they might even not work/not install, at all.
Go to this page,
and select to download only the BIOS and the Chipset drivers. Those are the ones most likely to install USB 3 capability. After installing those, if they even install... being for Windows XP, they might well 'choke' a bit or a lot... try
a restart of the Dell 9100 PC
then, see if a USB 3 memory stick, attached after that install of Chipset and BIOS from Dell, and a restart, might actually work.
Let me know how it goes, please. (If you need further assistance, you may also have me work on your PC remotely, too, for a small added fee, if you wish and so choose.)