From the Start Menu open command prompt and type:

sfc /scannow

Run CC Cleaner's registry repair.

If none of this works, try rolling back to an earlier System restore point where the desktop taskbar was present.

If none of this works, a repair or fresh reinstallation of Vista may be the only way to fix it.

EDIT booting to the Vista installation CD will get you to a command prompt/chkdsk and system restore function if the desktop is not capable of these functions. Booting to safe Mode is another alternative.