The answer is ‘it depends’ on the meaning you are trying to get across. Here are the relevant definitions from the New Oxford Dictionary of English (1998):

disorganized not properly planned and controlled; (of a person) unable to plan one’s activities efficiently.

unorganized not organized.

Disorganized is strong and disapproving; unorganized is mild and may be neutral. Compare:

Her desk and papers seemed unorganized (but she could find anything she needed in a moment).

Her desk and papers were disorganized (and she could never lay her hands on what she needed).

disorganized not properly planned and controlled; (of a person) unable to plan one’s activities efficiently.

unorganized not organized.

Disorganized is strong and disapproving; unorganized is mild and may be neutral. Compare:

Her desk and papers seemed unorganized (but she could find anything she needed in a moment).

Her desk and papers were disorganized (and she could never lay her hands on what she needed).

Possibly due to the French origins of the “dis-” prefix, disorganized seems more formal and “proper” English to me, but I really don’t know if there’s any tanglible difference between the two. As far as I know, its not regional.

I would say a thing never organized is “dis”, a thing degenerated or that can be improved by intelligence is “un”.

I would use them that way and that is personal preference. it’s a personal (or editing) choice. Ain’t grammar fun?

or·gan·ized (ôrg-nzd)

adj.

1. Functioning within a formal structure, as in the coordination and direction of activities.

2. Affiliated in an organization, especially a union.

3. Efficient and methodical.

And here’s the definition of disorgainized:

dis·or·gan·ize

Show Spelled Pronunciation[dis-awr-guh-nahyz]

–verb (used with object), -ized, -iz·ing. to destroy the organization, systematic arrangement, or orderly connection of; throw into confusion or disorder.

Also, especially British, dis·or·gan·ise.

So I’d say they’re both correct depending on the term you use them in!

…Hope this helps!!!=]

♥