Optiplex 755 USB problems

Hi,

I'm having a problem with an Optiplex 755 ultra small form factor PC not assigning drive letters to USB flash driives.

It originally had windows XP. I wiped the hard drive and did a clean install of Windows 7 Pro 32 bit and applied all Windows updates.

I updated the BIOS and installed the Intel_AMT-HECI_A02_R255437, Intel_AMT-SOL--LMS_A02_R255438, and Intel® 6, 5, 4, 3, 900 Series Chipsets updates. All devices are recognized and there are no question marks in Device Manager.

But USB flash drives are not being assigned drive letters.

It's a Sandisk Cruzer, 16GB, FAT32, which works perfectly in virtually every other PC I use it on. I've done a full CHKDSK on it. No problems. But this Optiplex refuses to assign it a drive letter.

I've seen these posts:
http://en.community.dell.com/support-forums/disk-drives/f/3534/t/19465285.aspx
http://en.community.dell.com/support-forums/desktop/f/3514/t/19380705.aspx
about the underpowered USB ports on the Optiplex 755.

So I tried using a powered USB hub. It makes absolutely no difference. Front ports, back ports, no dice.

I see "Cruzer" pop up in Device Manager, and if I'm using a powered USB hub, "Generic USB hub" pops up too.

But the flash drive is never assigned a drive letter.

Now I'm not 100% sure about this, but I'm almost certain that when this Optiplex 755 ultra small form factor had XP on it, it recognized this very same flash drive, without a powered USB hub. Now that it has a clean install of windows 7, it doesn't, even with a powered USB hub. Another Optiplex 755 small (not ultra small) form factor that still has XP on it also recognizes the very same drive on all ports, front and back. All the PC's are running the same version of Norton Internet Security.

Anyone have any ideas? Am I missing some drivers? How do I go about troubleshooting this?

Thanks in advance.