If you run Windows you can DIY from your desktop:
Start > Programs > Accessories > System Tools then
Disk Defragmenter/Scandisk/Disk Cleanup, etc in turn and let each do it's own thing. Monthly defrag is recommended. You can also set Maintenance Wizard to carry out these tasks regularly.
Invest in a virus protection prog. Norton AntiVirus does a good job
Hope this helps. It works for me and was highly recommended by the IT whizz-bangs at my pre-retirement place of work.
PS: Clear out your emails. Don't maintain a web history. Delete all temporary internet files (Start > Control Panel > Internet Options and follow the prompts).