Is it possible to run IIS7/7.5 on Windows Server 2003?
No, since IIS4, the IIS version has been tied to the OS version.
IIS7, specifically, has many hooks into Server 2008 features that don't exist in Server 2003. So even if you were able to force the installer to complete on 2003, IIS would not work. Upgrade to a newer OS, or live with IIS 6.