Slow laptop

My laptop is running windows XP, its about 4 years old now. It was working almost like brand new, never really had a problem with it until recently. It suddenly became slow and unusable. I scanned my computer for viruses/malware with my kaspersky anti virus, windows defender. I also scanned my computer with malwarebytes anti - malware. Nothing came up and everything was clean so I thought I would try Ccleaner, defragment, disk error check and my laptop still is unusable. I downloaded windows registry repair and it seem to work but every time I turn my computer on I have to scan it with windows registry repair about 2 or 3 times before I can use it. It's starting to get annoying having to do this all the time. I need help figuring out what exactly is wrong and how I can fix it. I'm assuming it's something with the registry but I'm not that good with computers and not sure how I can fix this. Thanks!