My computer is randomly slow down

Hello everyone,
Actually, I am doing simple things like browsing the internet and making some paper works in my computer. There are times my PC is getting lags, when this happen I can’t tab out my current windows and when I click Alt Ctrl Del the pop up message was appeared and it says “Preparing security options” then a few seconds my PC back to its normal speed. I already upgraded my system unit and it is all brand new. It worked well in the first month after that it slow down. I used different software to protect my PC such as registry cleaner, disc cleaner, disk defragmenter, memory check and I’ve done volume error checks on both my HDDs. Please help me.
