from a previous post on this same topic - https://answers.splunk.com/answers/173767/splunk-version-for-windows-server-2003-32-bit.html
I would suggest that they install the latest release version of the UF 32 bit 6.1.4.
“This version of Splunk is supported but is not recommended on this platform” - is only applicable to 32bit installs on 64 bit machines I believe; so not relevant in this case.
Please note that you only have about 9 months of support left on those. On July 15th, 2015 support ends on Windows Server 2003