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ShellRunAs is a very good utility authored by Mark Russinovich and Jon Schwartz that allows you to run applications under different user accounts. They describe their work as:

The command-line Runas utility is handy for launching programs under different accounts, but it’s not convenient if you’re a heavy Explorer user. ShellRunas provides functionality similar to that of Runas to launch programs as a different user via a convenient shell context-menu entry.

ShellRunas works on Windows XP, Windows Server 2003, Windows Vista, and Windows Server 2008.

You can download this utility here.