From the Start Menu open command prompt and type:
Run CC Cleaner's registry repair. http://www.piriform.com/ccleaner/download
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.