I'm having the same prob! it will let me open like exe files and a couple others but not zips,rars,aces etc. I have installed IE SP1 but I uninstalled it. Is this something that IE SP1 could have left behind?
The best way to find out what happend, is to compare the registry of someone that doesn't have this problem, with someone that has it, and see if there's any option missing for the Internet Explorer key or something like that.
I haven't noticed it did that for rar files too. Question. To all of you that have this problem, which version of WinRAR are you using?