Start > Run (WIN+R): regedit
"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\Advanced"
New > DWORD (32-bit) Value
"TaskbarGroupSize"
Double-click new entry and set value to "0"

This is the secret to making all the task-bar windows in Vista (and I believe XP) group together whenever there's more than one.  I am so much happier now that all windows of the same program group into one task-bar button before the entire task bar is cluttered.