Yes, with so little info in your post I'm going to guess you need to make a slipstreamed disc with Some files removed.
I had a post about this a few years ago. Google is your friend
you should find the solution in one or both of these two links (I succeeded after following these instructions) ->
http://glennstevenson.com/Blog/TabId/1115/PostId/4/Install-Windows-7-64-Bit-On-Mac-Pro.aspx
http://sergiomcfly.blogspot.ch/2008/04/select-cd-rom-boot-type-when-installing.html
Which drive bay are you trying to install to? If it's a separate hard drive then Windows often won't let you install unless it's in the first position of the SATA array.