Remember that Windows is not a sure thing about this. For example if someone installed some antivirus suite and it's blocking the install, we get to dig in and work around that. I've lost count of how many times I've uninstalled Norton and others to get this working.
Then again that new ZeroAccess is creating a lot of trouble.
I'll need more clues to offer anything other than try other browsers.
When I try to install some downloaded software,my computer says "windows installer could not be accessed. This can occur if the Windows Installer is not correctly installed. Contact your support personnel for assistance." Never had a problem before.