defrag hard drive on reboot or without logging in

We use Altiris network management software to do everything with our PC network. (imaging, patch management, etc.) The one thing we can't do is get the workstations to defrag remotely. We can power on the PC's remotely and run scripts and executables, but you have to be logged on as an administrator to defrag. I'm not familar enough with the defrag utility to understand the switches etc. to get it to run in a script on startup before loggon. We could even use AD GPO scripts to run it on startup, but my users don't have Admin local and if they did might be a little upset and waiting for a defrag to complete in the morning. I really can't afford a commercial defrag program like Diskkeeper but need to keep my machines clean. Any ideas, freeware, scripts, etc that might work would be appreciated.