I discovered an error with the Search Server Express 2008 installation process that I thought I would share.
I was installing this on an existing WSS 3.0 server. I had previously modified the server's web.config to include a defaultProxy element in the system.net section.
Once the Search Server Express installation process had completed, my main WSS site stopped working. Trying to access any page on the site would bring up an ASP.net error page.
It looks like the Search Server Express installation made an incorrect update to the web.config. I guess it wasn't expecting there to already be a defaultProxy entry. Once I removed the invalid elements the site started working again.
If you come across this error, I hope this post helps you get back up and running quickly. If you are running WSS and haven't installed Search Server Express yet - do it! Of course, you will get more out of it if you do some up-front planning.