This is more an answer than a question- When installing SCCM the installation failed and I was stuck in a situation where I could not uninstall the site or install it, the easy answer would be to use another server or rebuild it, but that sometimes is not an option. The resolution was to create a database with the expected name (see the SCCM logs for this) and then specify the primary server and the site code details that it's expecting to find (you can find this in the SCCM wizard logs). You will need to setup columns, i've provided a screenshot. You are essentially fooling the uninstall checker to think there is an sccm database there, with the details it wants. You then run the splash.hta, select uninstall and go back and forward in the dialog ( this is if the next option is greyed out the first time you come to it) and you then can uninstall the primary site and a cleanup will be done.
Note- that this is when the installation had fallen over in a heap and the option to uninstall was not there in the Add remove programs etc..
Also the error message is a bit misleading, what it's essentially saying is that it's detected an installation already, but you are in a situation where you can't remove the installation to then perform the installation again.
screenshot is from my personal lab : /answers/storage/attachments/73431-sccm.png