Run .msi as administrator

runas /user:domain\administrator "msiexec /i C:\application.msi"

Windows XP

Run cmd.exe process as System Account in Windows XP

1. Start > Run > CMD
2. at 11:20 /interactive "cmd.exe"
   

You must have administrator permissions.
Set time 1-2 minutes from now.
At specified time cmd.exe (or any other application) will start with SYSTEM permissions.

To view all tasks
at

To delete all tasks
at /delete

This only works in Windows XP, for Windows Vista and Windows 7 check this article.