Serious issue in installing SP6

I have a really serious issue in installing SP6. Actually everything had been working fine since I deployed 7.4 SP6 last week. But today when everyone tried to tried to install SP6 by launching the point, they got the error saying
"This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer package."
I only have one way back now which is SP5. But even while installing that I got the same error. I tried many alternative ways to do that like with cached installation, without cached installation and non- cached installation but in this case the error appears even before it downloads. Please help me out in this matter I would be very grateful.
