In linguistics there is a term, register, for the formality level of speech or writing.
Installers or “Setup” programs – at least on the PC/Windows platform – are traditionally upper register, using formal terms. I suspect this is because the common installer making programs (InstallShield, WISE, etc.) supply most of the boilerplate text, such as the introduction page, the scary copyright warning and similar text.
One place that is left up to the person making the installer is the system components and requirements specification. For Civilization III (a good strategy game, by the way) the person responsible got a little loose:
Kind of obvious here, since you almost never see it, the unit MB is “spelled out” as “megs”. While pretty nearly universal in speech, it is almost never written that way.
Aside: You don’t run across megs/MB very often now, since so many things are either too large (gigs/GB) or too small (kilobytes/KB).