The problem was that there WAS 4gb of ram but somehow one piece of 2gb wasn't connected to the motherboard properly (I wonder how that happened). That was the actual reason of lag caused on my system.
After I found that out I managed to connect it and then everything was back to normal.
And after all, I thank everyone who engaged himself to help solving my problem.
So, I have noticed that my computer started to lag about 2-3 days ago. It is mostly visible when I play a game and battle is loading for two minutes (it is used to be about 20 seconds). When battle finally starts there is constant lag (not from internet connection).
I have installed CCleaner from Piriform and cleaned all the junk files. Then I installed Deffraggler also from Piriform and defragmented my Local Disk (C on which I have stored WoT.
Problem was not solved, there are still massive lags and lond loading time.
Processor - AMD Phenom II x2 555, RAM 2GB, Graphics card - ATI Radeon HD 4800
Please help as soon as possible.