I have built an updated web installer which should allow us to replace just the one file on the server to fix the issue. The only downside is that it is not compatible with existing installs of 3.5.3 (that is, you can't run the executable to Modify/Repair/Remove an existing install, but unless you've corrupted your machine you can do those through Programs and Features).

If anyone would like to test it, I've attached the file here. I've done one test on my own machine (separate from the build machine) including installing the launcher and all optional components and it was fine, but some more validation before I replace the existing executable would be nice.

(Added RM Larry as FYI. This doesn't affect anything else.)

AFAICT so far, it appears to have been a build glitch where the launcher MSI was rebuilt in between 32-bit and 64-bit builds (the weird part here is that the rebuild was correct and the initial build was wrong, which is backwards from what I'd expect...). The upload included the rebuilt MSI but the bootstrapper was expecting the initial build.

I'm still trying to track down why that happened, but in the meantime I've added a test (PR 258) to my upload script that will detect problems like this before we announce the release.