What can you use on Windows to try out Android apps you find online? What can you use to quickly try out apps you find online? 

The tool should


Run on Windows
Give you root access so you can run apps that require root access
Have functionalities of a physical android device. In other words things like GPS and a Internet connection should be present and you should be able to install apps using APK files or through the official Google App Market. 


These things would be nice to have but one can still work around them


The tool runs fast and lag free
It's lightweight and can run smoothly on some of the most low spec hardware by today's standards.
You can use Android Device Emulator for this. It is packaged with The Android SDK and a virtual mobile device that runs on your computer. The emulator lets you develop and test Android applications without using a physical device.

Link: http://developer.android.com/tools/help/emulator.html