Note on node title:
I wasn't sure if I should've named it just with 'overbar' or 'overline', but then I thought if somebody knew it as an 'overline', they might end up missing the entire node.
Also I wasn't sure if I should include the 'Microsoft Word' part, because I wanted somebody to append to this node with LaTeX formatting, but usually the more specific the node title is, the better(?) Also I'd like to do it in HTML if a way exists.
I will assume you are either using MS Word 97 or 2000 (I haven't tried it in 95).
What is an overbar/overline?
It's a bar you draw on top of something. My Prof in Hardware Design calls it an overbar and I've seen people on the net call it an overline. When you do so, you 'complement' an object in your equation.

1 = 0
Read this as "The Complement of 1 is equal to 0."
This is useful in
1's complement and also,
2's complement.
How to do an overbar/overline on a letter, word, digit or number?
1. Fire up MS Word
2. Go to 'Insert' on the menu bar.
3. Select 'Field'
4. A window should pop up. Select 'Equations and Formulas' in Categories.
5. Choose 'Eq' in Field names.
6. In the Description field, type the following:
EQ \x\to(DESIRED TEXT HERE)
where DESIRED TEXT HERE is obviously the text you want to enter.
For example:
EQ \x\to(0h f5cK 1m 4 m1mb0!)
outputs:

0h f5cK 1m 4 m1mb0!
How do I do a doubleoverbar/overline?
In short, the same way you do an overbar/overline, except you have to enter the same equation (minus the EQ) within the brackets.
EQ \x\to(A)
The complement of A.
EQ \x\to(\x\to(A))
The complement of the complement of A.
What if I want to include brackets in my equation?
Use the escape character '\':
For example,
outputs:

(A)
Other things to know
 Use 'Toggle Field codes' (by rightclicking on the equation) when you want to edit an equation without going through the window prompt.
 Double click on the equation if you want to use other equation/formula functions. Note: I haven't figured out how to change the font from this prompt.
 Sometimes when you enter a symbol (such as the symbol for intersect or union) you will get a weird character different from the desired. In those cases, copy the desired symbol or character you wish to output and use the 'Toggle Field codes' function and paste it manually.
Update: I have since converted to using
LaTeX which proves to be much more effective at producing overlines. FYI, overline{
argument} is how you would do it. Simple
innit?