PowerBuilder 7 is not "supported" at all any more. The Sybase End-of-Life for PowerBuilder 7 support was July 15, 2003. So Sybase no longer certifies it for any version of Windows. (FYI, the End-of-Life for PowerBuilder 8 support is April 2, 2005).
As to whether your application will run under Windows 2003, the answer is: It depends. If it's a basic client/server application (not a distributed app), uses only basic external API calls, and does not use a lot of third party DLLs, then it will probably run OK. Otherwise you will probably run into a lot of trouble. The bottom line is, if it doesn't work, Sybase will not help you resolve the issues unless you upgrade to a later version.