Before switching the IIS from 64bit to 32 and vice a versa, Don;t forget to remove approprate asp.net runtime
using aspnet_regiis found at Framework and Framework64 folder. After switching the IIS from 64 bit to 32 bit mode run aspnet_regIIS found at 32 bit
framework. If you are switching from 32 bit to 64 bit apply aspnet_regiis found at Framework64 bit. 

   You can't run 32bit asp.net application on IIS running on 64bit mode.

To run your asp.net web application on IIs running on 32 bit mode you need to configure you IIS 7.

First you need to open your command prompt and execute below command to configure IIS configuration.

cscript.exe adsutil.vbs set W3SVC/AppPools/Enable32BitAppOnWin64  1

Here 1 refers to true.

Then you need to run aspnet_regIIS  -i   found under framework  v2.0.


Fore more information refer

http://www.revenmerchantservices.com/post/2010/07/30/Removing-aspnet-runtime-from-IIS.aspx

If you are still trouble shooting with IIS, post your thread at http://www.iis.net/ you can use your windows live ID to login or create new account.

 Here are some useful stuff http://support.microsoft.com/kb/911720

Satalaj