There are repair tools available for msi installer, but if your registry entries are corrupted, it is not easy to repair with those programs. You could also may not able to see it in plain english in registry. There are millions of entries regarding msi, and that path could not be easily accessible. The main source of installer , if corrupted, could not be repaired with available tools.
So, it is better that you attempt fresh install which will remove all the system agony.I have tried deeply in to this, by going to the registry entries, it will be in coded words, not in plain english and nothing worked for me. Registry repair tool should have worked well, it replaces the default entries for all the registry main and subkeys, but it is not possible to delete those entries, created by the infected programs. So those key entries are there to create more problems. Fresh install is the best possible, or otherwise, repair install would be the last option to try.