Assumes you have Windows 8 or later, which greatly improved the built-in accessibility features.

COLOUR

Turn on Windows Magnifier. Set it to 100% - that is, no magnification. But in Settings, check on "Turn on colour inversion". Your screen is now mainly white-on-black and less glaring.

Option: Instead, select one of the High Contrast themes. Not all applications will respect this, however (Chrome offers you a High Contrast extension, for example).

TEXT SIZE - GLOBAL

Reduce your screen resolution (not the text scaling, the actual number of pixels horizontally and vertically). This works fine in every application, where changing the text scaling doesn't work across every application. It's the simple fix I employ most often.

Option: also check out the Display > Change the text size only options, which give you bigger title bars and suchlike.

TEXT SIZE - PER APPLICATION

Yeah, this is the tricky one. You don't want to have to scroll left and right: as you say, "make things fit inside the screen".

Microsoft Office - zoom controls, bottom right of each application, combined with "Draft" or "Read" modes in recent versions to give you more text and less pretty whitespace on the screen.

Browsers - check for "Readability" functions, either built in or as extensions: page loads, you click on them, text all fills the screen, flowing and readable and sizable without left-right scrolling. "Reader View" is the one in Firefox, icon in the address box. "Reading View" in Edge, same effect. The Readability extension in Chrome.

Option: try using the mobile versions of websites, like mobile.facebook.com, which have simpler layouts assuming less horizontal space and therefore zoom better.

PDF - Adobe Reader - F4 to make PDFs reflow, normal zoom controls then zoom text without left/right scrolling.

NEXT STEPS

Your vision may degrade further as you age. Check out NVDA (open-source free screenreader) and WindowEyes (commercial but available for free for anyone with a copy of Microsoft Office).

I work in assistive technology and have developed the open-source WebbIE software for fourteen years for blind screenreader users: http://www.webbie.org.uk/

Fine motor control? So gross is okay, can move arms or legs in a big way, but not fine finger movement? The general term is "Assistive Technology".

Use built-in system adaptations: change mouse sensitivity, keyboard repeat rate, use the numeric keypad to move the mouse. See Control Panel > Ease of Access Center in Windows. "Make the mouse easier to use" and "Make the keyboard easier to use". http://www.microsoft.com/enabl...

Tremors? http://www.steadymouse.com/ to dampen mouse movement.

Move the mouse using a trackball, can't click? Dwell clicker. http://sensorysoftware.com/mor...

Could move a game controller or joystick, not the mouse? JoyToKey http://www.oneswitch.org.uk/2/...

Can move head? Cameramouse, http://cameramouse.org/

Not use a keyboard? Probably up to using an "on-screen keyboard" and "switching". There's an OSK in Windows, 7 and later is OK, before then not so good. Many others, The Grid 2 is probably the best. http://sensorysoftware.com/gri.... You'll find at this point that everything is starting to look very "special needs" - the market usually addresses people with cognitive as well as physical problems, and starts to get called "AAC". But the technology is in there. You might also want to check out switching with an iPad/iPhone - recent iOS releases have fantastic switching capacity built in. Proloquo2Go is the most famous iOS app. It's expensive for an app, but it's dirt cheap compared to dedicated hardware solutions (like Stephen Hawking stuff)

Operate one control only? http://www.webbie.org.uk/onesw...

In the USA? Try finding your state's Assistive Technology Resource Center. In the UK? ACE Centre is good, http://acecentre.org.uk/.

Key thing: usually people put off acquiring and learning to use the technology until it is too late, because it's too depressing. The medical channels for getting this stuff are often slow (at least in my country, the UK) so if your friend has a progressive, degenerative disease, you might be best going with something you can get right away and is not too off-putting - if you get an iPad and use that, you can get it right now and it doesn't have as much stigma as an obviously medical device. Many of these conditions have a very limited lifespan, so you need to get something soon if it's going to be useful.

It's also worth noting that switching is really slow and painful for someone who is used to normal usage, and that the role of the main carer/partner is essential in successful adoption of this kind of technology.

(Quick whirlwind notes from a technical rather than medical guy, excuse any slightly-off nomenclature. And your friend might just need to adjust her Windows settings, and I've leapt to much more "advanced" systems than she needs - but you don't think a trackball will cut it, and she's clearly been normal up to now, so I'm thinking the worst...)

We do a similar piece of software that tracks your face movement: it's free, and you can get it from http://www.facemouse.co.uk/

Install (Windows only), run, position your head facing ahead at the webcam, and then move the mouse around by turning and raising/lowering your head. There are two versions, one that click automatically when you stop moving your head and one that doesn't (so you can use another dwell program of your choice.)

evolution by definition increases the survivability of the species.

That's not correct. Biology is full of species that have evolved to fill particular niches, like the panda or flightless birds on islands. When the niche disappears the species becomes extinct. So evolution is perfectly capable of reducing survivability, depending on the timescale you're measuring and the area you're studying. Generalists survive, then specialise into the new, vacant niches in their local environment.