There is no word in the dictionary which spells 'atleast'. Infact as I write this the spell checker tells me this (and has been telling me this since a long time). It's just that I today took the pains to look it up in a dictionary. In fact the word 'infact' also does not exist.
And I have been using both the words comfortably since time immemorial. I did correct myself multiple number of times during writing but the words remain imprinted in my mind in the wrong formats. I today correct myself consciously. And henceforth will take care to think correctly when I use these in spoken language and not only the written.