well as usual... Make sure ur system is virus free..
u didnt maintioned the OS.. but as u write Windows Defender, i suppose its Windows XP...
now mainlly this error comes if the downloaded file is Corrupted... I suggest u redownload the file using any other Download manager than what u used 1st time to download... u may use FlashGet...
make sure ur Windows is Up to date and install Windows Installer 3.1 v2
and if the problem still presist.. u may try to look at the KB from MS...
You cannot install some updates or programs
hope this helps...