Because 10.2.0.5 is a Patch Set , not a standalone installer, the process differs depending on whether you are starting from scratch or patching an existing instance.
. You usually need to install the base release (10.2.0.1) first and then apply the 10.2.0.5 patch (Patch number Oracle Forums Recommended Alternative Oracle Database 10g Release 2 10.2.0.5 Download NEW