How can I make my mom’s Windows PC bullet-proof?
How do you set up your parent's computer(s), if you're the on-call computer expert ?
Do you recommend linux, firefox ?
Do you use vnc or equivalent ?
Ubuntu LTS with default set of software (gnome, firefox...) . Works pretty well as long as you don't tweak it too much... (I somehow feel that ubuntu scripts are very prone to nonstandard settings -- but I guess that's only my feeling). For support I installed openssh-server (they've got public IP address) and use either plain ssh or (for explaining them how to do things in GUI) x11vnc.
This works for me... but I know linux pretty well, I wouldn't recommend this for newbies.
But: they don't need MS Office/reading proprietary MS formats. If this is needed, I guess you have to install windows. All of solutions I know (wine, virtual machine with XP, openoffice) have drawbacks that make them unfeasible for common users.
I've got this working for last 3 years or so... I usually check it once a year.
I install Chrome, remove all shortcuts to Internet Explorer, and create a desktop shortcut to Chrome with IE’s icon. Works like a charm.
I currently use Windows XP, Firefox, AVG Anti-Virus, and sometimes Window SteadyState.
SteadyState was a late addition to the toolbox, and it keeps the computer running in peak condition all the time. Also, it would be a good idea to enable Remote Desktop so that you can help from afar, since I found it more useful than Remote Assistance.
The BOFH windows-version, slightly exaggerated for poetic beauty and attempt at being funny or confusing: I boot it over PXE using WDS, let it install the Win7 parents image I've prepared along with the required applications, then I move it to the parents OU and it will lock itself down accordingly and patch itself when needed... after those few minutes are done, I go grab a beer and continue enjoying the simple life as a former Windows sys-admin, reading some Vertigo comics perhaps.
Applications, I tend to go for defaults like IE because parents need something that looks familiar and running in the correct security context, with third-party tools like Flash and Reader patched, there's not much to worry about. My grandparents on the other hand are extremely tech-savy so they run whatever they want, and I don't manage their stuff, and it usually breaks frequently ;)
As I have an RODC at their place for login caches and simple file replication I use the existing ipsec vpn tunnel to their network and then either Remote Desktop or Remote Assistance when applicable to help them interactively. However, as it's Windows, that's rarely needed and most problems can be solved by a policy or a script.
Obviously, except for mother nobody has access to an administrative account, and hers only work for the clients at their house and is only used when needed - which so far is about once a year.
When they have a dynamic IP I let the firewall/router/NAT-device handle it and send updates to dyndns.org - and then let their endpoint establish the tunnel back. For a cheaper version you could just forward RDP directly to their computer - but running RDP over the internet without adding some sort of better security isn't exactly recommended.
For antivirus I've started using the beta of Microsoft Security Essentials (Morro) which seems nice and non-disturbing enough so far. For tech-savvy people like my grand parents I've recommended they buy into NOD32, which they have. They also need to run some anti-malware tools at times, because they're not the best at judging what is good and bad to do on the internet. Malwarebyte's Antimalware is my current choice.
I'd recommend going with what you are most comfortable with and knowledgeable about yourself.
That includes both the choice of operating system, the configuration of it and the choice of applications including the web browser. You know Firefox the best? Go with it. You've always been a windows-user? Go with that, and so on...
...remember that sometimes things like cheap printers, scanners and digital cameras have a very picky selection of drivers. Educate your parents before they buy, or you might end up having to go with the most mainstream choice.