The answer simply is that it is not included in any Microsoft Product.. You have to separately purchase and install these fonts @ 35 Euro's per font-style or $885USD for the complete set, the cost rapidly escalates
Arial and Helvetica are very close and only a few letters really show the differences. Microsoft has made arial an alias to helvetica since Windows 3.1, now the default since Vista is Calibri. This is because Helvetica is a copy-written font (designed in 1957) and is rather expensive to license. In order to keep the cost of the operating system down, Microsoft licensed some fonts from Monotype who designed Arial Sonoran San Serif, which was originally designed for IBM’s bitmap font laser printers. Ariel is that were very similar to the fonts people were used to. Helvetica was a very popular font used by typesetters and by typewriters. Courier and Letter Gothic were also very popular.
GQR1 are the famous letters for checking. If on a Windows machine you will have to look at images in order to see the difference, unless you decided to pay the royalty fee ( ~ $30 per Helvetica font)
Arial vs Helvetica
check out the fontometer
If you see Helvetica in the Font Selection and you did not purchase the helvetica fonts then Word is lying to you, you are not seeing Helvetica at all, it is really Arial and if you check out the font it will tell you that the font is NOT installed, and that windows will use what it considers a close font that actually is installed.
The real G has a tail going straight down almost to the baseline. The R has a little tail on the baseline at the bottom of the diagonal.
If you are curious about the fonts that are included with windows I direct you to this
Another misconception is that people believe that Helvetica is on a webpage and it looks so much better than Arial but the truth be told, they were actually seeing Arial
The CSS specification allows for multiple fonts to be listed as fallback fonts. In CSS, the font-family property accepts a list of comma-separated font faces to be used, like so:
font-family: Helvetica, "Nimbus Sans L", "Liberation Sans", Arial, sans-serif;
The first font specified is the preferred font. If this font is not available, the Web browser will attempt to use the next font in the list. If none of the fonts specified are found, the browser will resort to displaying its default font face. This same process also happens on a per-character basis if the browser is trying to display a character which is not present in the specified font.
how to spot arial http://www.ms-studio.com/articlesarialsid.html