Windows only: If a lot of un- and re-installs left your system a bit messy, Little Registry Cleaner can clean it up, along with startup items and other system utilities. Little Registry Cleaner's primary function is scanning your Windows registry for invalid entries. You can select which types of entries it will scan for, allowing you to be cautious and only scan for orphan program entries, but leave anything related to the system drivers or DLLs untouched. Little Registry Cleaner has a built-in backup system; once you've done a sweep, you can restore to the registry's previous state if you notice things have gone awry.
In addition to speedy registry cleaning, Little Registry Cleaner also has an uninstaller to ensure you don't end up with orphaned files and registry entries to begin with. You can also edit your startup items, as Little Registry Cleaner has a clean and easy to use interface for seeing which programs are set to start at boot and their associated registry keys. Previously reviewed and popular CCleaner has a similar set of features with a more fire-and-forget style interface, but Little Registry Cleaner is a tool for cautious, incremental cleaners. Finally, it should be noted that cleaning your registry isn't a magic potion for turning an aging cart horse into a a derby winner; it benefits you most when it removes entries that are causing buggy performance or misapplied software tweaks. As always, be cautious before rooting around in your computer's brain. Little Registry Cleaner is freeware, Windows only.