Problem with marquee tool

[sharlok]

Hi, I am using photoshop for last one year. But from a few days I am encountering a peculiar problem with my photoshop. When I select something by using the marquee tool (it can be any of marquee tools) it started fine but after some selection it automatically select the area and i have to start back from the starting point. I can't make out what actually is happening. I am here giving the specs of my computer and the screenshot of the performance window. I don't if they will be helpful or not.

My computer's spec:
processor -> Intel Dual Core 2.5GHz
MotherBoard -> Asus P5KPL AM PS
Graphics -> ATI READON 4650 HD
RAM -> 4GB DDR2
HDD => 1 TB

Screenshot of Performance


Please solve my problem because it's harming my work as lasso is a very important tool of photoshop and I can't use it.

[SeniorS]

I don't understand " after some selection it automatically select the area".
Can you make screenshot of before and after that error? Not sure it'll help but it will make thing easier to understand.

But FIRST:
If some tool start to behavior strange then ussually helps reseting PS preferences How to reset Photoshop preferences (In Windows and Mac)