Because most likely you have some **favorite tags** which might include [tag:windows-7] 

Then all questions about [tag:windows-7] get highlighted on the site for you, because you're likely more interested in these

See this MSO question for more information: 
[What do “favorite tags” and “ignored tags” do?](http://meta.stackexchange.com/questions/19173/what-do-favorite-tags-and-ignored-tags-do)

For future reference, [go have a look around the FAQ on MSO][1] it will most likely answer a lot of your questions


  [1]: http://meta.stackexchange.com/questions/7931/faq-for-stack-exchange-sites