2003 upgrade option not available

Am currently running 2003 Enterprise R2 SP2. 2008 tells me I cannot upgrade but have to do a custom install, why? The only thing which springs to mind is having XPPro dual boot from another drive, but why should 2008 even consider this? Is there a known work around? Thanks.[tfl - 05 12 09] Hi - and thanks for your post.You should post questions like this to the Technet Forums at http://forums.microsoft.com/technet or the MS Newsgroups at http://www.microsoft.com/communities/newsgroups/en-us/.