Yesterday I managed to take down my development web server. 

I changed one of my customer's web sites from an SQL database to Microsoft Access, and as I am running 64 bit Windows Server 2008 and there is no 64-bit OleDB driver for Access, I had to change the Advanced Settings in the site's application pool to enable 32-bit applications:

Unfortunately, I then started getting Service Unavailable Errors: HTTP Error 503. The service is unavailable on all the sites on my development box.  Changing the above 32-bit setting back to false didn't solve the problem.

Mike Volodarsky on his IIS 7.0 Server-Side blog has a great article describing the problems that I was having here and this is the first port of call for anyone having these same issues.  My own problem however, ended up being that IIS seemed stuck in 32-bit compatibility mode after I enabled 32-bit application support, which then caused all my IIS worker processes to fail (Not sure why though!)

To resolve the issue, I ended up needing to run the following script in a command prompt:

cscript c:\inetpub\adminscripts\adsutil.vbs SET /w3svc/AppPools/Enable32BitAppOnWin64 False

After I restarted my application pools, everything worked again.

Hope this helps,

Bartek