If you had a bad update, it is likely your registry is corrupt. If at all possible, I would recommend a Windows reinstall (it is quite likely other software has also been affected). However, in the past I have had similar problems and the Python installer does seem particularly susceptible to problems if corruption occurs (I resolved this without a Windows reinstall by uninstalling Python then deleting all registry entries related to it - though I would not recommend this more generally). I may take this up with Steve (in a separate issue) to see if we can make the installer more resilient.