I have IE9 and it works just fine on this system and I have IE8 on my XP Virtual Machine and it works fine too. What operating system and service pack are you using? What version of Internet Explorer? 32-bit or 64-bit? Beta or non-beta? More information would help. Are you sure it's the browser or maybe an add-on that isn't fully compatible perhaps?
I had a similar problem last year and some this year .This could be a problem with Mcshield.exe.When you run a scan open up task manager.See what is eating up resources.Make sure to click show all.Then run a scan and see what is going on.
My operating system is Vista Home Basic 32-bit, Internet Explorer 7, Service Pack 1. I have no idea what Beta means. I purchased McAfee Internet Security 2009 version(CD-Rom) and through auto updates, 2009 became the 2010 version then two weeks ago, through auto updates, I received the 2011 version. I never had a problem with the 2009 version but I have had alot of problems with 2010 and 2011 versions of the software. The software just keep getting worse with every new version.
It is most likely because Vista SP1 hasn't been supported by Microsoft or McAfee since July 12 this year. SP2 should be installed forthwith and all the dozens of updates since them.
There's help with that here: https://community.mcafee.com/thread/2019
Beta means pre-release versions of anything that are available for testing from time to time.