I cannot change the power schemes in Windows anymore. To see a picture of
what happens:

Clicking the drop-down arrow does not bring down any menu, not even an
empty one.

Strangely enough, "ACPI Multiprocessor PC" (I have a Pentium-4 with HT
enabled) does appear under Computer in device manager.

This may have happened after I upgraded my BIOS (downloaded from Dell, I
flashed it in Windows, and there were no errors or anything.)

Any idea what's wrong or how I can fix this?