why?
some parts of internet connectivity are tied into IE (such as PPoE and dialup connections), so reinstalling it may be necessary.
fixing IE
by the way, if you have such a connection and it's not proper for your internet service, remove it. it will block your ability to use the internet. Start, Control Panel, Internet Options, Connections tab.
another issue is proxy servers. if you used to use a proxy server and you don't now, remove it.
try IE again (unless you were getting "choose an application for iexplore.exe" - then just continue).
start, control panel, add/remove programs, add/remove windows components.
uncheck IE, click next...etc. until IE is uninstalled.
recheck IE, click next...etc. until IE is reinstalled.
everything in IE will be reset back to defaults if I am not mistaken.