Hi
thanks for sharing your problem with us, yeah i have faced that problem many times, it happens when RAM is full, and the virtual memory isn't working good, or it is sometimes, bcoz of over heat, or some times, when there is something loose in the pc, or sometimes, bcoz of software.
I would suggest you to install another operating system, i mean the same that you are having, but a fresh copy of it, then it will be ok . just try it out.
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Asfandiyar---->....~!
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