Groupwise typically will not install on Windows Server 2003 due to DEP incompatibilities. Data Execution Prevention (DEP) is a set of hardware and software technologies that perform additional checks on memory to help prevent malicious code from running on a system.
To resolve this issue:
1) Right click on My Computer > Properties
2) Select Advanced > Performance Settings > Data Execution Prevention
3) Select Turn on DEP for essential Windows programs and services only > OK > OK
4) Reboot the Windows 2003 Server
For more details on DEP, see MS KB 875352.