I got quite a mess after downgrading from 2009 SP5.1 to SP4.1.
I have lost several programs/folders from C:\Program Files (x86) after downgrading.
I run uninstall SW2009 x64 SP5.1 from Control Panel -> Programs and Features.
After that also installed SP4.1 and upgraded SW2010 to SP4.0.
Vista still starting but have to reinstall almost everything under 32 bit.
I hope this is only for me.
More info: Got an error message from another program: "The feature you are trying to use is on a network resource that is unavailable."
Edit2: Java Virtual Machine missing ...