Memory Usage Goes Up Gradually (Memory Leak)

When I starting the Windows (in the first few hours) everything is normal But after 1 day or 2 (sooner or later) the amount of available memory gradually goes down and the physical memory usage increasing. This increasing, continues until the memory usage reaches near the 88% and at that point , there isn't any memory available to operate any software . Also when memory reaches that point, I can't access any drive in the HDD; because everytime I want to open any folder, the following error appears :

c:\ is not accessible.
Insufficient system resources exist to complete the requested service

At this point, I even can't shutdown or restart Windows normally; I must restart windows manually by pressing the restart button. after restart, everything goes to normal again; but this problem appears again and again ... (I had to restart Windows every 1 or 2 days)

my system :
------------------------
OS : Windows 7 x86
Cpu : Intel P4 3.0GH
Vga : nVidia 6600
MB : Gigabyte GA-8I915G Duo
PSU : Corsair 750W
RAM : 2GB DDR-400 Transcend
(I have these memories for 2 years)

Physical Memory: 2000 mb
Paging File: 2000 mb
Virtual Memory: 6000 mb


I already did the following actions but unfortunately none of them helped me :

0- using some optimizing and freeing softwares like (Memory Booster Gold - RAM Saver Pro - CleanMem)

1- Scanning my computer with antivirus and antispywares

2- Changing antivirus software

3- Updating windows with all updates available via Windows Update

4- Updating drivers of all hardwares

5- Downloading and installing the following hotfixes that are related to memory leak :

kb976658
kb977357
kb978042
kb978155
kb979223



So is there anything that I can do to solve this problem ? any solution ? hotfixe ? trick ? suggestion ? anything ? ...