Windows Server 2003 R2 Service Pack version

Yes. The complete requirements sheet at microsoft site.

SQL Server Express System Requirements

Supported Operating Systems: Windows Server 2003 Service Pack 2; Windows Server 2008; Windows Vista; Windows Server 2008 R2; Windows 7; Windows Vista Service Pack 1; Windows XP Service Pack 2; Windows XP Service Pack 3;

Physical Limitations • 32-Bit Systems: Computer with Intel or compatible 1GHz or faster processor (2 GHz or faster is recommended. Only a single processor is supported.) • 64-Bit systems: Computer with Intel or compatible 1.4 GHz or higher processor (2 GHz or faster is recommended. Only a single processor is supported.) • Minimum of 512 MB of RAM (1 GB or more is recommended) • 1.9GB of free hard disk space

Check you have the correct edition of the service pack for the version of SQL Server you've already installed - there are 3. I suspect you're either trying to install the x86 SP on an x64 install or vice versa (I saw the same thing myself about a month ago when someone tried the x64 SP on an x86 install).