What if I ask for a software running on Windows.
For instance I use Windows 7 64 bit Home Premium.
That means I could run almost any software since Windows 95. How should I tag my question? Just [Windows]? Or be very specific?
Be broader in the tag than in your question. Let's say I want (using the example Gilles tossed out) something very specific that runs on Windows 2000. I would apply the windows tag to my question, then within the question itself:
This has to be able to run on a 32 bit Windows 2000 setup, with 8MB of Ram powered by hamsters fed intravenously from an espresso machine.
You don't want to deviate too much from the parent tag, lest you end up with:
Natural tag curating will turn up needs to apply more precision in the OS tags, I have a feeling Ubuntu might want to separate from Linux, or BSD from Unix, or a bunch of other possibilities. For now, lets keep them as simple as we can.
Go with the tag(s) that express your requirements.
Most software for Windows will run on Windows 7 64-bit Home Premium. So you should just go with windows, and the question will be equally applicable to anyone running most versions of Windows.
You should use a more specific tag if you have a more specific requirement. For example, if you have an old box running Windows 2000 that you're using to drive your $100k experimental setup, and you need software that's compatible with it, tag your questions windows-2000.
Use generic tags. These are for quick filtering. Specify the versions in the post body.
In the MS Windows example, this means that these are your tags: