Internet Explorer 8 can be installed on Microsoft Windows Vista Service Pack 1 (SP1), Windows Vista, Windows XP Service Pack 2 (SP2), Windows Server 2008, and Windows Server 2003 Service Pack 2 (SP2).
Note, IE8 is still in Beta stage so only download if you know what you are doing. As for myself I will continue to use Firefox and Opera as my web browsers.
Link to the Internet Explorer 8 Download Site
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You also might want to read "Why Internet Explorer 8 might not pass the Acid2 test"
The Web Standards Project (WaSP) announced the release of Acid3, the latest in a line of tests designed to expose flaws in the implementation of mature Web standards in Web browsers.
Acid3 Test Released