Alright Christy follow these steps
What you want to do to fix this error is buy this reflow flux
http://cgi.ebay.com/NO-CLEAN-SOLDER-FLUX-BGA-GPU-CPU-REWORK-REFLOW-REBALL_W0QQitemZ130379083458QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item1e5b32eec2
Now watch this video, you only need to flux the gpu.
http://www.youtube.com/watch?v=h2R_NqF5wAc
Now read this tutorial
http://xbox-experts.com/tutorial/heatgunning-with-isolation/
And watch this video
http://www.youtube.com/watch?v=MEvfBS6Tams
Now you have to copy the fluxing and the heatgun video. You want to flux the cpu and then use a heatgun to reflow the motherboard. You'll smell the flux when it gets hot enough, you want to be in a well ventilated area because it's not good to breathe in the fumes.
More info on your error
http://xbox-experts.com/errorcode/E08/0020/
Let me know if you have any other questions or want a professional reflow. I'm offering discounts to yahoo answers users. You can contact me via my yahoo answers profile.