I just updated to 2.1.2, but during the installation, it asked for the old installer for some reason. Luckily Sun is gracious enough to keep the older builds for download and I just did that and pointed the installer at the new location.
So why did it ask for this anyway? This is the first time this is happened, but it's happened with other programs and is extremely annoying. I assume it has something to do with the MSI installer, but why VirtualBox asked it is what I'm asking here.
When installing a newer version with the Microsoft Installer software, it will remove the old version using the information that can be found in the installer file. Thus it looks for it in your temp folder. A disk cleanup or other means could clean this folder and remove the install file.