If you're looking for a free way to optimise your PC, Comodo Registry Cleaner is a great bet.
Brought to you by the same people who make the excellent free Comodo Firewall Pro, this equally excellent Registry utility seeks out errors and junk in your Registry, fixes them, and leaves you with a more stable system.
Like other good Registry cleaners, Comodo Registry Cleaner does all of the work automatically. Run a scan, and it unearths Registry problems for you, allows you to review them, and cleans up the ones you want to fix.
Comodo Registry Cleaner has some nice extras as well, including the ability to create backups before cleaning your Registry, and the option for you to tell the program to perform scans and cleanings on its own.
In addition, a startup manager lists every program that runs when you boot your computer, and lets you stop those items that you don't want to run.
All in all, Comodo Registry Cleaner is a useful, neatly designed program—it's tough to find a better free Registry cleaner.