Office 2007 is 32 bit only so there is no issue there. Your problems arise only with Office 64 bit which has both 32 and 64 bit versions.
You cannot hope to support users with 64 bit Office 2010 when you only have Office 2007. The solution is to upgrade.
If the only Declare
that you have is that ShellExecute
then you won't have much to do once you get hold of 64 bit Office, but it's not really viable to support users when you can't run the program that you ship! Just think what you would do you do when they report a bug?