Try running this to ensure that its not a virus thats already on your system preventing you from installing an anti-virus:
http://housecall.trendmicro.com/
if that doesn't fix it
do a system restore back a few days to a time before this started to happen.
http://housecall.trendmicro.com/
if that doesn't fix it
do a system restore back a few days to a time before this started to happen.