Hello,
Sollution:
Right-click on your desktop, then click on Personal Screen Properties (dont
know what it is in english, got an Dutch Vista).
A screen should pop-up and now click on the left side of that screen on
something like Change Desktop Pictograms.
From here you should be able to get your Trashcan back.
Again, my Vista is a Dutch version, though i hope this helps you out!
Regarts