Memory Leak?

I see the commit charge only 400M but I can't open internet explorer or any other application, I have 768mb memory, it happens a lot of the time usually after 3-5 hours of turning pc on.

Open your Task manager and then open a good representation of the programs you normally use.
Double the number of windows that you would normally have open and running just to be on the safe side.
Check the Commit Charge Total (CCT), Commit Charge Peak (CCP), and the Physical Memory Total (PMT)

Please report back those values.
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Have you run your antivirus and antispyware utilities to rule out malware?
Have you used Disk Cleanup to dump Temporary and Temporary Internet Files?
Have you Defragged your hard drive?
How much Free Space do you have on the hard drive?
Have you opened up the case or Tower to blow out the dustbunnies?

Download Everest 2.20 (free) here.
http://www.majorgeeks.com/download4181.html
Run Everest
In Everest - Navigate to Computer - Sensors
Create a Quick Report of the Sensor information (include the Temperatures and Voltages, but omit the several "blocks" of random number information below those informations)
_________

Download CodeStuff Starters, here:
http://www.snapfiles.com/get/starter.html

Run CodeStuff Starters
From the Starters(tab) - GoTo - File - "Save as Plain Text"
Include that Plain Text Report in your next Reply in this thread.
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In your next Reply include:

Values requested from TaskManager
Answers to the listed questions
Quick Report from Everest - Computer - Sensors
Plain Text Report from Starters

That will give us a better idea of how to assist you.

Best Regards

661620 current 741420 peak 784368 memory total