I was recently attempting to install McAfee Antivirus install on a PC and it would not complete due to an error stating that it was not a valid executable file.  During one of the many attempts it also stated that the “Installation failed, please contact your network administrator”.  The PC was running very slow and I attributed this to its age and RAM.  After further investigation it was discovered that there were over 18,000 folders in the C:\windows\temp directory.  After deleting these folders the machines performance improved greatly and the Mcafee install went through the first try.   Don’t forget to try clearing out your temp folder if you are experiencing performance issues or an installation problem.