I have configurated several legacy ASP websites in my IIS 6 server, together
with new asp.net 2.0 application. For stability sake, a separate application
pool is built for those asp applications. However, we met the session state
loss problem when running these apps.

To address this problem,
1) disable the recycle time setting in application pool;
2) set timeout after idle to 20 mins;
3) make sure web garden to 1
This is only for in process mode. Out-of-process mode is not affected.

For the underlying mechanics, please refer to David’s blog at
http://blogs.msdn.com/david.wang/archive/2005/09/19/Why_do_I_lose_ASP_Sessio
n_State_on_IIS6.aspx