The Windows Registry is a major cause of system slowdowns, and can potentially lead to system crashes and conflicts. When you install or upgrade software, that software writes information to the Registry. So the more software you install and uninstall, the more clogged your Registry becomes, and the more problems your PC may have. Sammsoft's Advanced Registry Optimizer 2010 ($30, limited demo) peers into your Registry, uncovers problems, reports on what it finds, and fixes them for you. It also backs up your Registry, so that if any problems occur after cleanup, you can easily revert to the previous version of the Registry.
It offers as little or as much customization as you want. You can, for example, use the one-click option that scans and fixes all problems. Or you can instead ask to see what problems Advanced Registry Optimizer finds, and then have it fix only selected ones. If you're not familiar with using a Registry tool, your best bet is to go with the one-click option, and not bother with customization. You can also tell the program to only examine certain portions of the Registry, and ignore others. It also includes a Registry defragmenter.
There is one problem with the program: its cost. At $30, it's expensive, especially considering that a program such as donationware CCleaner includes a Registry cleaning tool. Advanced Registry Optimizer 2010 offers more customization and control than does CCleaner, so if that's important to you, you may consider paying $30. Otherwise, you'll do well to use CCleaner, which has other system-cleaning tools in addition to a Registry cleaner.
The trial version of Advanced Registry Optimizer 2010 will only clean up 100 Registry errors. To fix the rest, you'll have to pay for the full version of the product.