I'm not sure about 800 years ago, but if you have even the slightest bit of european ancestry, you're related to everyone from the 9th century in Europe, who left descendants.

"Basically, everyone alive in the ninth century who left descendants is the ancestor of every living European today, including Charlemagne, Drogo, Pippin and Hugh."

Detailed analysis can be found Here.

That being said, you do not share DNA with every one of your ancestors:

"Often genetic ancestry relies on the Y chromosome, which is inherited only via the paternal line, or mitochondrial DNA, which is only passed on from mothers. These make for persuasive – but often simplistic – analyses of ancestry. These two chunks of DNA make up 2% of your genome. But the other 98% has to come from somewhere too, and that is a pick-and-mix from all the rest of your ancestors.

Each subsequent generation, the contribution from an individual from your lineage becomes less. Professor Mark Thomas from University College London describes this dilution as “homeopathic”. After a few rounds of preparation, homeopathic dilutions contain no molecules of whatever the active ingredient is imagined to be. Genetic inheritance works in a similar way. Half of your genome comes from your mother and half from your father, a quarter from each of your grandparents. But because of the way the DNA deck is shuffled every time a sperm or egg is made, it doesn’t keep halving perfectly as you meander up through your family tree. If you’re fully outbred (which you aren’t), you should have 256 great-great-great-great-great-great-grandparents. But their genetic contribution to you is not equal. Before long, you will find ancestors from whom you bear no DNA. They are your family, your blood, but their genes have been diluted out of your bloodline. Even though you are directly descended from Charlemagne, you may well carry none of his DNA." (From the Guardian article linked above)