Is there something like Things for Windows?
The closest one I've found is Doit.im. But I've heard that it is unstable and its website seems outdated.
Try Nirvana. It's web-based, rather than desktop-based like Things, but is very similar in UI, and, since it's online, can be accessed from anywhere. As someone who simultaneously uses PCs and Macs, I long since ditched Things for Nirvana.
I suggest GeeTeeDee. It's simple but well-polished.
Try using evernote ...synchs with everything and is searchable...