I get asked about slow PCs fairly often, and the thing that always strikes me is how consistent the advice the owner has already tried is. Defrag the drive. Clean the registry.
Turn off visual effects. Delete temp files. Fire up msconfig and start disabling services until the PC feels snappy again.
Someone they know as "the tech person" said to do that, or they typed the problem into Google and got the same list back. It's a checklist of advice that's remained more or less unchanged for nearly twenty years.