In ASP.NET 2.0 you can use the built in Health Monitoring Events to log application restarts along with the reason for the restart.  To do this you change the master web.config file in the C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG directory and add the following to the <healthMonitoring><rules> section

<add name="Application Lifetime Events Default" eventName="Application Lifetime Events"

                    provider="EventLogProvider" profile="Default" minInstances="1"

                    maxLimit="Infinite" minInterval="00:01:00" custom="" />

http://blogs.msdn.com/b/tess/archive/2006/08/02/asp-net-case-study-lost-session-variables-and-appdomain-recycles.aspx

Otra forma 

cscript adsutil.vbs Set w3svc/AppPools/DefaultAppPool/LogEventOnRecycle 255