Malware has grown to epidemic proportions in the last few years. Despite applying layered security principles, including running antivirus, antispyware, and a firewall, even a careful user can fall victim to malware. Malware-infected downloads, drive-by exploits of Internet Explorer (IE) vulnerabilities, and a careless click on an Outlook attachment sent by a friend can render a system unusable and lead to several hours with the Windows setup CD and application installers.
One of the most effective ways to keep a system free from malware and to avoid reinstalls even if malware happens to sneak by, is to run as a limited user (a member of the Windows Users group). The vast majority of Windows users run as members of the Administrators group simply because so many operations, such as installing software and printers, changing power settings, and changing the time zone require administrator rights. Further, many applications fail when run in a limited-user account because theyБ─≥re poorly written and expect to have write access to directories such as \Program Files and \Windows or registry keys under HKLM\Software.
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.
Read more here.