I will shamelessly plug a little tool I wrote for entering symbols in Windows as I find any solution usually presented too cumbersome for daily frequent use. My personal use case is typing the Swedish å for example on an international US keyboard without having to switch layouts.
It allows the entering of unicode characters through a popup window not dissimilar to how this works in Apple OS X.
See https://github.com/mjvh80/SymWin for details, it's free and open source, but must (currently) be built, if there is sufficient interest I could add a pre-built version.
The tool can be configured per key, e.g. by copy/pasting symbols once from a site such as http://copypastecharacter.com. It comes preconfigured with various symbols.