There are very, very few acceptable uses of & in proper written English. Here are some of them:
& is especially common when joining names to indicate a firm or a partnership, for example, a law firm:
Baker & McKenzie
Abercrombie & Fitch
Crosby Stills Nash & Young
In abbreviations, when abbreviating "and", & is often used:
AT&T (American Telephone and Telegraph)
P&L (profit and loss)
R&D (research and development)
One rare usage is on envelopes addressed to a couple:
Mr. & Mrs. Jackson
&c. is a rare and somewhat archaic looking abbreviation for etc.
Other than that it is vanishingly rare to see & in formal written English, although of course in informal email, text messages, notes, and handwriting, anything goes.
Short answer: you can't do much harm in using guarantee every time.
Longer answer: the New Oxford American English presents “guaranty” as only slightly different from “guarantee” in meaning, and goes further to list “guarantee” as a variant spelling for “guaranty”. In addition to the information research in Fowler (quoted, for example, here), it seems safe to say that you can use “guarantee” all the time without fear.
Best Answer
I would suggest that inspite — as written in your question — is not in fact a word.
I think you must mean in spite of, which is directly interchangeable with despite.
I am not aware of any real difference between the two options, though I tend to use despite purely for efficiency of words.