To see if you are related to a "famous" person, you need to compare your ancestry to their ancestry. (usually by comparing your ahnentafel to their ahnentafel).
The best source for ahnentafels of American presidents is "Ancestors of American Presidents", by Gary Boyd Roberts, which you can find in just about any library of any size. The most recent edition, I believe, has Obama's ahnentafel in it. These ahnentafels only go back 6 generations, as I recall; so will not always take you back to American immigrants of the 1600s for recent presidents. I compiled my links, shown below, by looking for my ancestors in the Index - note you kind of have to memorize your ancestors - which is a challenge when you have close to 150 separate immigrant ancestors identified back to the 1630s.
For the ahnentafels of other famous folks, you can refer to "Notable Kin", also compiled by Gary Boyd Roberts. There are a couple of editions of this work - again, can usually be found in most sizable libaries (or purchased). Again, look for your ancestors in the index of each edition.
He also has published numerous other articles in periodicals of famous folks related to Princess Di and to Winston Churchill. Again, look for your immigrant ancestors in the ahnentafels of these two, or the famous folks he has tied them to.
The most frequent success with this kind of "searching" is usually resereved for folks with New England immigrants of the 1620s - 1640s. They go back the furthest.
As an example, see these pages:
http://freepages.genealogy.rootsweb.ancestry.com/~riss/baldw...http://freepages.genealogy.rootsweb.ancestry.com/~riss/baldw...