There are some simple tips to keep your PC clean. Here are some tips:
1. Keep your recycle bin always clean.
2. Cleanup your disk regularly.
3. Clean your PC's junk files once in every week. (How to clean junk files)
4. Optimize your PC.
5. Keep 20% of your hard disk free to have a fast PC.
6. Remove desktop wallpaper.
7. Use analog themes and disable animations.
8. Disable unnecessary startup programs.
9. Don't keep many files or shortcuts in your desktop.
10. Do not keep unnecessary and unwanted programs installed.
11. Do not install any downloader in your PC.
12. Use cleaner software to improve performance.
13. Bring up task manager and end all unwanted processes. (Do not end system related processes)
14. Do not keep your PC on for a long time (3-5 hours)
15. After playing a high specs game, restart your PC.
16. Restart your PC after any installation process.
17. Scan you PC with antivirus software often.*
18. Some times anti-virus software may slow down your PC, so disable them. (Can be harmful)
19. Open RUN and type "tree" then press enter.
20. Update your display drivers regularly.
21. Do not run many processes at a time.
22. It is always good to free up some spaces on your HDD.
*Some times malwares and viruses may slow down your PC. So, scanning your PC and removing them will make your PC fast.
These tips are really very useful. By maintaining them your slow PC can become a super fast one! And it is also not necessary to have a beastly PC at all. Your simple PC will be able to handle your works.