Do you have rights to write to this directory "C:\Documents and Settings\[User]\Local Settings\Application Data", test this by creating a folder in that directory.
Run Build 226 installer again, does the same error occur?
Test installing Omniscope 2.6 b225, does the same error occur?
Just wanted to inform you I met same issue for installing b229 today (see attached error message). The download was fine but I had to run it twice to install