it looks like the error is
GetPreviousSettings: Error: DetermineContextForAllProducts failed witht: 0x65b.
Apparently you have some products on the machine which failed to be installed or uninstalled properly. The Splunk installer will attempt to find other Splunk instances on the machine, thus it needs to enumerate installed products. If it fails to read installer database in the registry, the installation fails because it cannot determine if previous Splunk product is installed (this is need to properly upgrade Splunk).
I would recommend trying to clean up installation database. For example this utility may help:
http://support.microsoft.com/KB/290301,
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