Windows ASCII Character/HTML Entity/Unicode
W in the Marlett font has the benefit of giving the user the Windows logo version of the machine he is currently using. For instance, on Windows 7:
⊞ (U+229E): ⊞ or bold ⊞.
However, Microsoft has been changing the logo every few versions.
For Windows Vista/7 it looks best in bold italic ⊞.
2021 update: For Windows 11, the symbol is perfect without formatting!
As explained in @HansPassant’s answer, there is really no adequate way to use the “Windows logo” (more properly called “Windows icon”) as a character. Theoretically, you could use a Private Use codepoint for it, but then you would need a font containing it in that codepoint, and of course this would not work reliably; Private Use codepoints are by definition for use by private agreements and should not be used in public information interchange (though “icon fonts” do just that).
And as mentioned in that answer, the Wingdings font has a “Windows logo” at code point U+00FF, where the letter “ÿ” should be present. This means that you could use
<font face=Wingdings>ÿ</font>
or, equivalently,
<font face=Wingdings>ÿ</font>
Naturally, in any system where a font named Wingdings does not exist, the user would see the letter “ÿ” in the default font. This also happens e.g. in Firefox on Windows, as the browser apparently refuses to play such a font game. Arguably, that’s the correct behavior: if you ask a browser to show a character, say “ÿ”, in a specific font and that font does not contain a glyph for that character, the browser should decline and use another font.
Company logos are not included in the Unicode character set. For obvious reasons, beyond those companies changing their logo all the time. Like Microsoft did with theirs. You'll want to avoid the marketing department of a company from instantly obsoleting your product :)
You can find non-standard glyphs in one of the symbol fonts, those that map glyphs that are not in the Unicode character set. On Windows, the logo is included in the Wingdings font, character code U+00FF. Use the Charmap.exe applet to see it. It is the olden logo, not the current rectangular one. Could still be appropriate, the key cap on the user's keyboard might still be the old one as well. Or in the way a computer user still knows that the icon for the Save command is a floppy disk, even though nobody has ever seen one in the past 10+ years :) If you put it on a web page then Apple and Linux users are of course unlikely to be happy campers and liable to leave quickly.
A Charmap.exe screenshot:
