I may have solved the problem... at least for now.

I downloaded the Windows 10 upgrade ISO image from Microsoft on a working PC and burned an install DVD.

I started my PC with locked-out USB ports with the DVD I just burned.  Some patience is required, as the boot-up just sits at the cyan colored logo for a few minutes without any whirling circle of dots to indicate anything is happening.

When the installer launches, select your language, then next.  On the next screen choose Repair My Computer.  Choose advanced options.  Choose Restore an earlier configuration.  When System Restore launched, no restore points were shown, so I clicked Show more restore points.  One point labeled Direct X Installed was displayed, so I selected that and continued.  Restore showed a moving progress bar for a few minutes and then displayed a modal dialog saying an unknown error occurred and my files were not changed.

Out of curiosity, I rebooted Win10, and the mouse and keyboard were working again!

In Windows 10, I checked the device manager and found two Generic USB hubs listed under Universal Serial Bus Controllers.  Under the Power Management tab of Properties for the hubs, I removed the X mark  from the "Allow the computer to turn off this device to save power."

I have my fingers crossed that this was the issue.

I had to back out of a W10 upgrade for same reason no USB ports would work.

ASUS advise no newer USB driver - in fact they aren't even producing one for W10.

ASUS dod advise that there are new drivers for:

Chipset - Intel Chipset drivre

Chipset - Management Engine interface

"Perry Kincy" you mention an ASUS  driver that increased performance is the issue - can you be more specific so I can remove it before I try upgrade again.

As frustrating as that is, it appears to be the only viable solution for many.  Once you get back to Win 7 and can again use your computer, you can visit the Dell boards to find out what setting or driver in your system needs to be fixed/replaced to be compatible with Win 10.

Many of us had Asus MBs and that turned out to be a USB driver that increased performance that had to be disabled.

Hopefully you will find the Dell solution with the help of other Dell users.  For sure MSFT isn't much help.

I did a fresh install of Win7 Pro on a Delll T110 mini tower, then upgraded to Win10.  The installation worked OK for a few reboots, long enough to get to the point where the massive build 1511 (?) update installed.  I have not installed anything other than the O/S because I have not had the opportunity.

There aren't any PS/2  ports on a T110, so USB keyboard and mouse are mandatory.  There are four USB ports on the back and two on the front.  Windows appears not to recognize anything plugged into any USB port.  The seascape picture on the login screen is pretty; I've spent a lot of time looking at it.

The T110 is in BIOS mode, not UEFI, and everything is properly enabled in the setup screen.  Obviously, the keyboard and mouse work in the BIOS.  There doesn't seem to be any way possible to press F8 before Windows [apparently] disables the USB ports.

Windows 10 has essentially turned the PC into a very expensive digital picture frame with seascape picture and handy digital clock.  Apparently, going back to Win7 is the only viable fix.