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.

Using ShellRunas
Usage: shellrunas [/reg | [/quietreg ] | /regnetonly [/quiet] | unreg | [/netonly] <program> <arguments>

/reg Registers ShellRunas shell context-menu entry
/regnetonly Registers Shell /netonly context-menu entry
 Note: a command prompt will flash when the program starts
/unreg Unregisters ShellRunas shell context-menu  entry
/quiet Register or unregisters ShellRunas shell context-menu entry without result dialog
/netonly Use if specified credentials are for remote access only
<program> Runs program with specified credentials and parameters

http://download.sysinternals.com/Files/Shellrunas.zip
http://technet.microsoft.com/en-us/sysinternals/cc300361.aspx