Regardless of whether you're using PHP, ASP, ASP.NET etc there is no native support for transactional deployment on IIS.
The simplest approach would be to have two physical folders and (optionally two web sites - one production, one test) on your web server, for example:
c:websitesmyappdep1
c:websitesmyappdep2
Initially your site would have its physical path pointing to c:websitesmyappdep1
.
When you deploy your latest build you'd deploy into c:websitesmyappdep2
. Once you're done just switch the physical path of the production site over to this folder. This means you still have the original site and can fall back to it if the new code fails for whatever reason.
The next time you do a deployment you'd deploy into c:websitesmyappdep1
and once you're done switch the production site to that folder.
You could optionally have a test site that points to the folder you're deploying to so you can make sure the site works before switching your production site over.
This could all be scripted.
Here's some related reading that may be of interest:
Publishing/uploading new DLL to IIS: website goes down whilst uploading
Is smooth deployment possible with componentized ASP.NET MVC apps?
Rob Conery also had an excellent blog post about the lack of a decent deployment story for ASP.NET application. You should take a trawl through the comments some of which are quite insightful:
ASP.NET Deployment Needs To Be Fixed
Getting Constructive On ASP.NET Deployment
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