Hi ian par,
What operating system are you using?
What Service Pack is currently install?
Are you current on all your windows updates?
Have you tried checking permissions within the Installer folder? To do this try the following below:
In Vista click Start Search > type > c:/windows/installer > then right click on c:/windows/installer at the top under programs and click properties > Click on the security tab > Verify if System and Administrator account has Full access permissions > The Everyone account should have read and execute permissions, List Folder Contents, and Read
Does your permissions have these exact settings?