Question, Are you positive it is Avast ?
I had this same problem on 2 computers on a network of 13 machines. All had Avast on them, but only 2 of them freaked out. ALL computers are the identical image, Windows XP Pro, Avast, Internet Explorer 8 only (not FF or anything else)
On one of the computers i reverted to Explorer 6, the problem went away. I put I.E. 8 back on the computer and the problem showed right back up. Looking at the page code, it appeared that not all of the HTML page code actually made it through transmission. Chunks of page code were not where they were supposed to be and that is what caused the page to nt display properly. Yes, hitting reload corrected the problem.