I have downloaded IE8 without any problems but use Firefox most of the time. There's a compatability view button immediately to the right of the address bar in IE8 - if you click on that, it might improve some of the problems people seem to be having.
If you're an XP user and want to go back to IE7, you'll need to uninstall IE8 via your Control Panel, then re-download IE7 from the Microsoft site
HERE. You might also need to adjust your automatic Windows Update settings otherwise you might find IE8 being installed again whether you want it or not