C Cleaner can remove a lot of bulk rubbish from HDD, get it below 70% capacity to work at its best. Depending on the OS you may want to defrag it too.
MalwareBytes (free edition ) will, well remove most if not all malware she might have picked up – 99% of slow PCs we get in are down to that. If it finds anything continue to keep running as many times as it takes to come up clear.
PC Decrapifyer will remove unwanted programmes and blotware.
Try to get as much dust and fluf out of the cooling vents with a vacuum – more effective than most people think.
That’s all free software – in our place (it support) we use the commercial versions of those and together with a single, good AV consider hardware to be “as good as it’ll get” after all that.