I was just going through the setup newsgroups and noticed some questions about 1618 failures.
A 1618 failure occurs when another setup package using the windows installer is running. Since the Windows Installer supports only one set up running at any given time, launching a second set up package will result in a 1618 error to the second set up package. The first setup package will still continue to run.
Visual Studio's setup itself is a bootstrapper. That means it chains in a bunch of MSIs (individual setup packages). A 1618 error may occur during VS install because as VS finishes up one setup package and then launches another one, the gap in between the two setup packages may occasionally be enough time for Windows Installer to start installing a patch it just downloaded. It's a race condition.
We saw a lot of these types of failures internally because our IT department pushes down patches when they're available. If you happen to be installing VS at that time, the patches will try to install the first chance it gets which could be in that little windows when the setup packages finishes and a new one starts up.
The workaround for this is to wait a few minutes for the patch to complete and then rerun setup or install the failed component manually from the \wcu folder off the root folder.