Blank screen after window loading indication

whenever I start my CPU, it runs on its normal speed for a while till Windows xp loading (processing) indication comes. After that the screen turns blank/black and the CPU becomes unbelievably slow. I encountered this problem yesterday. I have heavily invested in my computer and is afraid of any loses that may occur due to this. can u please suggest me a way to get my computer back on its normal speed. I would also like to bring it to your notice that the last my I used my computer, it shut down on its own while playing a movie and made a constant beep which indicated towards RAM issue. I removed one RAM of 2gb and then restarted the computer. it still has a 2 gb RAM but the problem of screen going blank and very very slow speed started thereafter. I had used my computer with the present 2 gb RAM many times, but it never had caused any trouble before. what to do?
 
Can you run a disk check by going to Computer, right-click on the C drive, select properties, tools and check now. Put a check in both boxes to set a disk check on next start up...
 
the last time I patiently waited for the desktop to load it took me ages. Also I was able to check the hardware properties of my computer: it showed nothing under the Computer title..... and before that it showed the RAM of 128 mb (only). Could it be a problem with the "windows"? The windows I installed were 100% genuine. If there was a problem with RAM, the computer should not have loaded in the first place. Also I tried to take some images in a pen drive which took me 2 hours to complete the total procedure. I am sooo confused. I am not in a habit of seeing my computer so dull and dead.
 
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