IMHO, the only way of returning it to its ex-factory condition is to do a complete fresh reinstall of Windows. That will remove all the crap. If there's anything you want/need to keep, back it up to an external drive first.
You can try downloading/running CCleaner, and this will undoubtedly be helpful. What it won't do, though, is to remove all the user documents that have piled up over the years. Perhaps try CCleaner first and, if you don't see the improvement you're looking for, go for the reinstall.