I've made some changes to machine.config which didn't work out as we
expected. When I changed back to the original version, none of the sites
work(including reporting services). I get an "Access is denied. " exception
with the following message:

ASP.NET is not authorized to access the requested resource. Consider
granting access rights to the resource to the ASP.NET request identity.
ASP.NET has a base process identity (typically {MACHINE}\ASPNET on IIS 5 or
Network Service on IIS 6) that is used if the application is not
impersonating. If the application is impersonating via <identity
impersonate="true"/>, the identity will be the anonymous user (typically
IUSR_MACHINENAME) or the authenticated request user.

To grant ASP.NET write access to a file, right-click the file in Explorer,
choose "Properties" and select the Security tab. Click "Add" to add the
appropriate user or group. Highlight the ASP.NET account, and check the boxes
for the desired access.

I ran aspnet_regiis -i, but it didn't fix it. Will a re-install work?