I understand that the Earth was a computer for deriving the Ultimate Question, (for which the answer was 42), and that calculation would take 10 million years.
What doesn't make sense to me is, once that computer was designed by Deep Thought and built by the mice/Magratheans, why any outside interference (such as Ford Prefect) were allowed on the planet and thus might disrupt the processing on it.
Because nobody was looking after it.
The Magratheans went into hibernation after the Earth was made and so weren't in a position to protect it.
The mice, being pan-dimensional creatures were too busy on the 5D TV Chat Show Circuit while they waited for the result - as stated by Benjy Mouse in Fit The Fourth of the radio series.
Nobody else knew it was a computer, it was just a planet like any other. Indeed, that is the beauty of the thing.
Nobody actually knows the Earth isn't a real planet.
Since the building of the Earth, the Magratheans had long ago gone into their self-imposed deep sleep and had faded into myth and legend. Most people in this enlightened age don't even believe that they exist, except for highly gullible people like Zaphod Beeblebrox.
As such, the purpose of the Earth has no doubt been long since forgotten, and as it looks like a planet, and moves like a planet, and acts like a planet, it might as well be a planet for all practical purposes, which makes it fair game for hitchers of all persuasions.
A fair question at this point would be, why didn't the mice construct some kind of shield to keep people out? The answer of course being that the only thing that sort of thing would accomplish would be to make people that much more determined to get in to see what all the fuss was about. And that would rather defeat the purpose entirely. Plus, that sort of thing would require a tremendous amount of power to keep going for ten million years, and who do you think wants to be responsible for that electric bill?
The start of the first book hints about why:
Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue-green planet...
It appears that the planet-like computer was placed in an obscure and untrafficked part of the Milky Way galaxy. From this we might infer that they simply did not expect anyone from the outside to come upon it, and therefore left it to its own devices. This is careless and sloppy thinking of course but nonetheless not impossible.
Also do note that in the narrative the Golgafrinchans caused the demise of the original cavemen of Earth fairly quickly, the latter starting to die out within only a year of the former's arrival, which is hardly even a shake of a lamb's tail in the ten million years the program was running. So even if the mice were keeping an eye on it, unless they did so meticulously and with utmost scrutiny then it is not hard to understand how this glitch in the input data could have gone unnoticed.
Based on the fact that the Magratheans intentionally placed the Earth into a relatively well trafficked region of space, it would seem likely that a small volume of alien visitations ("teasers" and the like) aren't just tolerable, they're part of the normal lifecycle of a normal planet in the Milky Way galaxy.
The alternative; placing the planet into deep space or hiding it between galaxies would likely result in the inhabitants becoming warped and unpleasant, much like the inhabitants of the planet Krikkit or prone to self-destruction like the Golgafrinchans
What the mice couldn't have predicted is that instead of an inconsequential number of alien visitors dropping by (advancing the planet's technology and generally prepping us to become part of the wider Galactic community) a whole damn colonisation fleet turned up and killed the most second most advanced inhabitants.
