Basically, you (or the software you were installing) switched your IIS site where OWA is installed into either a 32bit mode or ASP.NET 1.1. Since Exchange 2007 Outlook Web Access 2007 only runs on ASP.NET 2.0 in 64bit mode, you need to fix it back.

First, disable the 32bit mode for your web site. By default OWA goes into the Default Web Site context (0) so the following will take care of that:

cscript C:\inetpub\adminscripts\adsutil.vbs SET W3SVC/AppPools/Enable32bitAppOnWin64 0

Second, register ASP.NET 2.0 as the default framework for that web site:

C:\Windows\Microsoft.NET\Framework64\v2.0.50727>
aspnet_regiis.exe -i
Start installing ASP.NET (2.0.50727).
……………………………….
Finished installing ASP.NET (2.0.50727)

Restart IIS and you should be all set.