The family name originates in England, the spelling has many many variations.Mainly because the census, birth, marriage, death and parish registers were all given verbally and the register or census taker copied the information, and spelled the name as they thought it was to be spelled, and also because of errors in transcription.As an example my own great great grandfather's name is spelled differently on each census record, his birth registry, marriage registry and death certificate.It isn't uncommon to find spelling changes even between father and son.
If you are interested in a detailed search specific to the Hillsdon family, there is a myriad of information out there, once you know where to look.I would be happy to help you, or answer any questions you might have if I am able to.
I would beware of the places that say they will do a history search for you, or provide your coat of arms etc.There is a coat of arms for several Hillsden/Hillsdon familys, however just because it is there, doesn't mean that you are entitled to it. By that I mean you may not be decended from the people the coat was issued/registered to.The places that will do a surname history, again do a generic surname history.While there may be some sort of nobility suggested, as is the case with my own surname, it doesn't mean that you are actually decendants of those individuals.The companies that sell those type of things will never tell you that.
Sorry I digressed, back to the history.The earliest record of my surname, which is rather interchangeable with the one you are looking for in this instance, was in Buckinghamshire before the Norman Conquest.
The specific history of the "Hillsdon" surname really depends on the actual linage, there were also a great many people that changed their names in order to avoid things, and/or changed countries.Some moved voluntarily because it gave them the opportunity to be land owners, some were banished (mostly for religious reasons).
Anyway I hope that helps a little.If you have any questions I'd be happy to try to answer them for you.