What operating system and service pack, if applicable, is this? Also what version of Internet Explorer is installed, whether or not you use IE (McAfee does)?
You might want to download Malwarebytes Free from the last link in my signature below and to keep it free, simply don't accept the free trial offer.
Run that and see if it finds anything.
Thanks. Finally got it to install. An 'sfc /scannow' seemed to do the trick.
It would be more helpful if mcafee told you why it couldn't do the install rather than just refusing to do it!
I agree. Glad it's OK now.