I was using laptop for gaming initially and everything was fine. After some years i noticed slow down - everything was merely always loading. Game that once was loading fast started loading longer. I dropped gaming, reinstalled Windows and started multitasking, doing work on laptop like Word, Excel, surfing internet, listening to Winamp, using Dropbox sometimes, i use Atomic Alarm Clock, antivirus MSE and Evernote. Still my system was loading longer than expected. I am talking about applications, web browser tabs, new browser windows, clicking on link on website, opening windows explorer i.e My Computer. MSE antivirus update takes constant loading of hard drive which lasts for 3 minutes! (But what is surprising is startup of Windows' applications is pretty fast...)

I don't think it is swap file, i have over 500MB of RAM free and slow down happens very frequently. Look at this screenshot:

Take a look at processes running:

Here is my laptop specifications:

Intel Centrino Duo T2400 1.83Ghz

1GB of RAM (512MB Samsung + 512MB PQI)

80GB Hard Drive Hitachi Sata 150

GeForce Go7400

Mobile Intel 945PM Express Chipset

Should i benchmark my hard drive?

This post might help you.

No in my case hard drive is accessed when i take action, not when idle. And i don't have viruses and such stuff.

I was pointing out how to find the root cause. The way to find it is the same.

Yeah but processes are not the ones that slow down hard drive, files are. I need to locate files that are excessively accessed when i do take actions. Where can i find program which could tell me which file/s being accessed and how much data is being read/written to?

If a new install of your OS didn't work I would take a look at the health of the Harddisk; bad sectors (and faulty RAM) could mess up a PC good.