my notebook suddenly becomes slow and unresponsive but normal CPU usage and memory

my notebook computer turned very slow two days ago. I've been trying everything i know to try to fix it but to no avail. I've defragmented and used the error-checking on my two hard drives, done a full scan with avast - antivirus, i've tried in safe mode, downloaded malwarebytes and scanned, used the disk cleanup, downloaded HD Tune and run a test, i've increased the virtual memory above recommended levels, but still only helped to some degree. and CPU usage is normal and, even though it's using a lot more ram memory than my previous notebook on win XP, it is still using the same amount as before the computer got slow. I don't know what else i can do so i'm turning to others for assistance.
first i thought it had something to do with google chrome, and i actually found out that i had two flash plugins installed but after disabling the standalone chrome flash the computer was still slow.
I consider the notebook fairly new since i bought it about 5 months ago. so i don't think has anything to do with age.

For clarification is the PC slow off-line as well as on-line?

From the sound of it you are basing PC slowness from browser performance. If you only have issues inside a browser, that narrows the possible problems down a bit.

well, before i started to to try to find the problem the computer was slow offline too. it took forever to go through folders, and it was impossible to use vlc media player. but after i did all the above mentioned stuff it is now much better, although not as fast as normal.
but all the browsers are still acting slow or just unresponsive..

How big is the HDD on it? How full is the HDD? Have you defraged the HDD?

the hard drive is about 470 GB all in all. C is about 80GB and D is about 390GB. both have 60%+ free space. I have defraged them several times..