After you click the OK button you should get a screen, suggesting that you run the installer with logging enabled. Can you please, click that button and send us the log after the installer fails?
The log file will help us identify the reason for the error.
Just in case, can you check if you have the latest Windows Updates installed? KB931125 contains an update of the Root Certificates, which is needed by the MSHelp3 system for the verification of the Telerik Documentation.
the Telerik team