|I'm guessing that you installed IE8 most recently and it decided to overwrite the default browser settings. So I would personally go back to my favoured browser and re-apply the default settings - this will get rid of any overwrite that might have occurred. |
It could even be a problem with IE8 itself! Remember that it's still a Beta at this point and is littered with small bugs. I haven't heard of a default browser hijack bug, but it's not unreasonable.
And yes, unless you're a web developer, that really is a lot of browsers to have installed on one PC!