Dell SK-8115 USB Keyboard recognised but not working

Hi, My original PS2 keyboard died and I obtained a new Dell SK-8115 USB keyboard and installed it - or rather I finally did after solving all the HID problems which occured and reinstalled the files and activated the HID service, which took me almost a week to figure out!

Unfortunately, whilst the keyboard works during POST as soon as operation is passed to Windows it freezes: the NumLock light stays on but there's no reaction to pressing any of the keys. I've tried uninstalling/reinstalling everything, even configuring it as a PS2 keyboard, with no success. The only time I thought I solved the problem was when I inserted the USB cable into one of the two USB slots on my Dell screen, the NumLock, Scroll and Shift keys worked but then froze. I tried unplugging/reinserting/rebooting as USB/PS2 configuration but now the the result is the same whichever USB port I plug it into.

I've disabled and reinstalled all of my USB entries in Device Manager but this has made no difference: I still can't use the keyboard. However, I've come across a reference in the MS site, I think it is, which advises that Windows disables USB legacy support for 32-bit USB drivers to work, which would seem to explain what's happening. But this doesn't fit with the fact that all my other USB hardware works!? Their advice is to clear the Use Automatic setings checkbox in the PCI to USB Host Controller and to use the settings provided by the BIOS. However, this presents me with a bit of a problem as my PCI to USB Host Controller already shows settings for Memory Range CFFFB600-CFFFB6FF and IRQ 21; with the Use automatic settings checkbox ticked but greyed out and I can't access it to try if this is the solution.  This is driving me absolutely nuts! Can you help please?