Don't use "heavy weight" Anti virus / firewall / etc

Why?

Because they really make your computer slow! Did u remember the first time u buy your computer? It's really fast, right? Until u install many protection softwares. U need to replace your Heavy Antivirus into the lighter one. Avg is better than Kaspersk* or nort*n anti virus. Check out at avg.com for more info (and it's free!!!). Don't forget to contact me online with your messager at FIX_SLOW_COMPUTER@YMAIL.COM if u need my support.

And also firewall... Windows firewall is enough I think.

Use deepfreeze if u want to test a program, virus, suspicious exe(s), and doing something "bad" with your computer. And when u restart your computer, every trace, programs, configurations that u made before are gone forever. Save the necessary files into your Flash Disk or hardisk portable or online storage.

But remember, sometimes malwares (bad virus, trojan, etc) could slow down your computer... so, scan regullary your computer to fix your slow computer...