Process Explorer is one of several extremely cool tools made by Windows uber-guru Mark Russinovich.
In a recent blog posting, he explains how you can use Process Explorer to run specific applications as a Limited User, without the attendent hassles of actually running the entire user session as in Limited User mode.
An alternative to running as limited user is to instead run only specific Internet-facing applications as a limited user that are at greater risk of compromise, such as IE and Outlook. Microsoft promises this capability in Windows Vista with Protected-Mode IE and User Account Control (UAC), but you can achieve a form of this today on Windows 2000 and higher with the new limited user execution features of Process Explorer and PsExec.
Process Explorer’s Run as Limited User menu item in the File menu opens a dialog that looks like and acts like the standard Windows Run dialog, but that runs the target process without administrative privileges: