If you're configuring BAS on BizTalk Server 2004, and you have the Whidbey version of the .NET Framework installed, you might get an error during the configuration wizard similar to this one:

Error encountered while querying virtual server state for URL:
http://<server>/sites/BASSite
Error Detail: Could not load type 'System.Int32' from assembly 'mscorlib,
Version=1.2.3400.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

To fix it, go to your BizTalk installation folder, and create a new configuration file for ConfigFramework.exe and force it run with v1.1 of the framework:

<configuration>
 <startup>
  <requiredRuntime version="v1.1.4322" safemode="true" />
 </startup>
</configuration>

Then, run ConfigFramework.exe again and you should be able to complete the installation of BAS.