"Windows Installer Service could not be accessed"

Lately I've been running loads of updates for all sorts of fun vulnerabilities. One of the goofy errors that I've come across is that the "Windows Installer Service could not be accessed" and your software, especially your MS updates, do not want to install. Seems to be XP specific thus far. Probably rare in the "real world", but you never know what you might turn up. So I've found that running your command prompt in XP:

Unregister and re-register the Windows Installer

1. MSIEXEC /UNREGISTER
2. MSIEXEC /REGSERVER
3. Try your Windows Installer-based application again.

from http://support.microsoft.com/kb/319624.

-Isaac