The first advantage is the Visual Studio IDE. Have you worked with it ever.. If you havnt, then i am sure you'l find its so easy. the tasks that will require so much coding effort in PHP can be done in VS with just drag and drop or following som wizard step.
The next advantage is that you use the .net framework. so the libraries and classes are already tested and secure , so the chances are less that you will make some mistake as it is already done for you.
The third advantage is the flexibility of language. you can use VisulaBasic.Net or VC#(which is almost java) in your application..or even both..
Fourth Excellent support and forums.
You can begin exploring ASP.NET by downlaoding some video tutorials from www.asp.net . This are excellent resource for the beginners. I think there are around 100 of them there.
So to summarize i would like to say that asp.net is far easier to learn, very robust, and it can everything PHP can do with much less effort.
the downside can only be the OpenSourceFactor. .Net framework as you know is not open source like PHP.