If it were truly a problem only within avast and IE7&8 have a large user base these forums would be buzzing with activity; so you wouldn't have had to add your post to one that is over two months old. Given what you have said I believe there is some form of conflict going on, usually with another security program/AV or remnants of one, but that really should be dealt with in a new topic entirely of your own, to avoid any confusion in this topic for an old version.
I don't use IE as my primary browser, but when I do have to use IE8, I don't appear to have any problems with ie8 and avast 5.0.677.
How avast interacts with browsers would be the same across XP, Vista or Win7 should be the same, so I don't see how upgrading to win7 would resolve the problem (even if you could afford it).