"Does it happen in linux and mac os as well?if so why?"
Slow down is less apparent but ram and running as few programs and services are the Key.
Blam!
"It happens due to too many programs using RAM, you should delete unused items in the registry."
NOOOO to dangerous as you dont know how skilled the user is. Install cc cleaner and use that to stop start up programs. Remove temp files and clean the registry.
www.piriform.com/ccleaner
Then use this defrager to defrag and optimise the drive.
www.auslogics.com/en/software/disk-defrag/
Both are free.
The plop
"Every time you use a registry cleaner you keep your fingers crossed and pray."
I use ccleaner about 40 times per week and have never had a problem.
Of course your anti virus will slow your pc down.
Use Linux based system, No registry problems, De frags drive all time, No virus No malware therefore no Anti virus is required.
How to get started with Linux
Easy with no partitioning of drives.
Use the wubi ubuntu installer if using xp, vista or 7
www.ubuntu.com/download/desktop/window…
OR
use puppy linux as a frugal install
Down load a puppy iso from the links below
Read about frugal installs below.
No need to partition drives as that's risky until you have some experience.
OR
Usb
http://unetbootin.sourceforge.net/