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 SymWin for details. It's free and open source, but it 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 Copy Paste Character. It comes preconfigured with various symbols.