Upon booting up my PC this morning - not having loaded any
new software or made any other significant changes to my
system - I found no Start button/task bar/system tray at
the bottom of the screen. The alt-tab method of moving
between applications also is out of commission. I've
encountered no other problems or error messages.

I thought at first that I might have inadvertantly moved
the task bar or hidden/minimized it, but that's easily
solved with a few clicks and drags and it turned out not
to be the case. Has anyone else run into this? Any help is
greatly appreciated.