Download same version from top-10 results for "putty homepage" returned by your favorite search engine, and compare them. If they are not all completely the same, abort the installation. Otherwise, install it (from any of downloaded bitwise-identical copies, of course).
If you need more security, interpolate results from different search engines and increase the number of downloads.
Simple, effective, and usually works quite well for such low amount of effort.
To be more sure, download source, have several top-notch security experts, cryptographers and programmers audit it all (including all libraries as well as compilers), and pay the cost -- and probably still be hit by next heartbleed bug.