ecase

Recently, my emachine laptop started slowing down and I got lots of "OS not installed", and blue screens. And of course, I cannot locate the original cd that came with it. I defragged, cleaned up disk, removed all tmp files and even restored to previous point. I get a more stable environment, but still extremely, extremely slow. How can I fix this? It happened 2 other times, and I took to Best Buy and paid them to fix. They reinstalled OS and it cost me $200! I have copied all my docs, pics thankfully. I am trying to not have to take it to someone again, cause money is not available this time. Please help