I don’t actually like the phrase ‘optimizer’, as the result of using such programs seldom can be measured. That being said, I’ve never had problems after running CCleaner.
CCleaner (formerly Crap Cleaner) is a small utility that removes unused files from Windows. It can also scan and fix the registry. ‘Fixing’ the registry means removing unused or obsolete entries. Although I recommend to backup up the registry beforehand (it’s an option in the program), I’ve never had any problems.
CCleaner can also be used to start the uninstallation of programs and delete entries from the startup of Windows. I do recommend Autoruns for the latter, as programs can easily be enabled again with checkboxes.
A full list of features can bee seen here.
The program has a good rating from both editor and users at Download.com.
Another program that has the same functionality as CCleaner is Glary Utilities.