PsTools : Psexec - Windows Installer errors

I am trying to install an MSI file remotely using psexec. It works about 75% of the time, but 25% of the time the MSI comes back with error 1601 or 1603. I've tried the msiexec /unregister and /regserver tricks and restarting the Windows Installer service to no avail, restarting the computer doesn't work all the time. After much troubleshooting, I found out that it seems to give me these errors when there is no user logged into the machine (the machine is sitting at the ctrl+alt+del screen). When I get either error, if I go and log into the machine and try running the script again, it seems to work. 

I'm running it from a Windows 7 host machine, the clients are XP or Win7.

Any ideas?

Thanks