when installing "Agents for Visual Studio 2012" (vstf_testagent.exe) on Windows XP the installer tries to download and install ".net 4.5". this fails as 4.5 cannot be installed on windows XP. error code is "0x800713ec - Asia".
in turn the test agent installer terminates.
workaround is to manually install ".net 4.0", first. after doing so the test agent installer no longer tries to install any .net framework prerequisite and continues installing the actual test agent.
however, this clearly is a bug in the installer logic that should get fixed.