I need help or need someone to talk me through it I am a rookie and have a old computer that has alot of bad stuff that needs defragmenting and I cant afford to buy it,helpl please.

i was just wondering how i can check that there is no bad stuff stored on my computer and how i can clean up my hard drive so there is more space on my hard drive and not any unwanted files.

also

what files that are already stored on my pc are ok to delete?

My computer is slow. My virtual memory always says its slow. I have a virus protector that i quarintine the bad stuff ever once and a while. I want to set it back to factory settings. Will that delete unneccessary stuff? and make it faster? Can i do this w/o deleting some program like word? and powerpoint? if not are there any tricks to making it faster!?