Computer problems :(

Hey guys, i know i haven't posted in a while but i need help. Hopefully you guys can help me figure out the problem
So a couple of days ago, my computer shut down while i was working. I tried to start it up, boot windows and then it just shut down at the windows screen (windows7, where the 4 lights join together). Sometimes it started up, but the windows components kept on shutting down (i.e. "windows media player has stopped working" and then the "send error to microsoft", also happened with explore.exe). After going to have people see it, they said it was the PSU and the hard drive.
I bought and replaced MANY computer parts including the:
PSU
Hard drive
Graphics card
I was still having the same problem (the new hard drive just wouldn't install ubuntu, as i don't have a cd of windows (normal crashing as explained above)
I then replaced the RAM and it worked!!!! I was able to boot into windows, and everything was working. BUT after like an hour, it just stops and freezes. I can still see what i was doing before it freezes but the only way to make it unfreeze is to restart it. (sidenote- pressing scrollock and numlock doesn't do anything when it's in the frozen state)
I have no idea what to do, and i really need help
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FOR PEOPLE WHO DIDN'T READ THE WHOLE POST, HERE'S A SHORTER VERSION
windows kept shutting down before i could do anything (i.e. where the four win7 lights come together to form the windows logo)
I replaced the RAM, and now about every hour or so the computer freezes
ALSO- i can't get my windows 7 iso that's on my computer to burn onto a cd. imgburn says that the disk is not empty (it's a new pack) it's a dvd+r. If you guys can help me with this issue, it would help also
Guys, thanks for reading this. I just want my computer fixed, hope you guys can help

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