ACT 2005 Install fails on SQLHotfix818.exe on Small Business Server 2003

This problem showed up while I was migrating a client onto a new SBS 2003 server. The ACT install would just about complete, then complain about an invalid exception in SQLHotfix818.exe at the very end. Subsequently trying to start ACT would cause an "Object reference not set to instance of object" error. I tried a lot of things, but here's what ended up working:

1. Go to Add/Remove Programs and remove any references to MS SQL Desktop Engine (including SharePoint and SBSMonitoring, you can reinstall these if you need them. If you actively use either, make sure you have a backup!).

2. Rename C:\Program Files\Microsoft SQL Server to something else, so the ACT install can't find it.

3. Make sure the SYSTEM user has full access to the C:\ drive (per ACT KB article #16478)

4. Make sure all services that start with MSSQL$ are stopped (Start -> Administrative Tools -> Services)

5. If ACT is installed, uninstall it.

6. Now install ACT, and you should be good to go.