Couple of things you can do after installing 2003.
Boot from the W7 dvd select repair my computer and do a startup repair, most of the time this fixes the bootloader for W7, but then 2003 may not boot now. So then you need to install a BCD editor software in W7 to get both of them booting properly.
http://neosmart.net/dl.php?id=1
If startup repair does not get W7 booting again, there are some commands you can do from the W7 recovery environment to get the job done, (boot from the W7 cd, select repair my computer, then command prompt)
http://support.microsoft.com/kb/927392/en-us
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