Well, everyone's lagging ever since the server issues lately. That could be it, or maybe your computer is just not compatible. My friend's laptop does something similar, I am afraid I have not the knowledge of how to fix it. Good luck!
If you're still lagging, you can always turn off your water reflection by going to the Log-in screen, click Options, Video, then scroll down until you see Water Reflection and uncheck the box. You can also lower your view distance which is right under Water Reflection where you just scroll the bar all the way down.
About the blue screen of death, however, I do not know but I hope you get that figured out soon.
Hope this helped.