Error 1500. Another installation is in progress. You must complete that installation before continuing this one.

Symptoms

Cause

1. Multiple instances of MSIEXEC.EXE are running Installation routines for programs can call multiple copies of MSIEXEC.EXE, but only one program can call MSIEXEC.EXE at a time. If program "A" has called MSIEXEC.EXE, program "B" may not use the MSI installer until program "A" has released its lock.
2. Registry key that indicate an installation is in progress wasn't properly removed by MSIEXEC.EXE When a program calls the MSI installer, MSI adds a flag to the registry to indicate it is currently running. Any subsequent MSI installer processes check for this flag, and, if present, exit with error 1500, as it believes there's another installer currently running. If MSIEXEC.EXE isn't currently running, it indicates that this key was left behind by the previous installation.