aspx is a asp.net web application file. Generally this file is not present in windows. The only possible reason why it is present could be that either you have downloaded it from the internet (or some program from the internet which is a asp.net web application) or you have developed it yourself.
If it is downloaded, it cant be run until you have the corresponding class file with the extension aspx.cs. If it is present you have to go that folder where this file is present and open it in web browser. At times you may also require dotnet framework to be installed (1.1 or 2.0 or 3.0 or 3.5 based on the version on which the development was actually done).
If it is developed by you (which appears unlikely), then install framework, create virtual directory using IIS and right click the file in IIS and select "Browse"
Hope this will help.....!!!!