Depends on what you are actually looking for, because diagnostic software comes in all shapes and manners. From general clean-up to more commercial fault finding at a price.
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You could browse click here or click here for other ideas and solutions, both sites are very good. A Google search will provide other programs, but be aware that some free scan offers, may result in finding a number of problems on your computer. According to the scan, you may have to subscribe to the service for removal of the problems shown. My suggestion is don't bother, save your money, there's plenty of freeware out there, that will do the required job.