Both my parents gone. Mom never used computer, Dad did since DOS days for his business, every day. As they age and it gets more difficult, they simply just quit using it. After enough frustration in trying to remember, they give up. Before that happens they start answering spam and sending you some of the spam they get wanting to know if it's OK, or if you can help them achieve whatever the spam is about, etc.
Also did this walk with a neighbor I personally had taught in her early 70's even went to store with her to help her pick out one that would serve her in dialup days, and also allow card and board games to be loaded for her husband to play. As she reached 80, she just quit due to frustration. Before that she "traveled the world" on her computer, very happy to visit places and travel sites online and pretend she was there. Massive stroke took her within a day a couple years later.
It will NOT get easier for you.
Some have set up ageing parents with Remote Access where they can adjust and correct things from a distance on their parent's computers. Getting into their webmail accounts and training the SPAM filters is advised also. The best is AOL mail where you can use a positive only filter, which means no email other than those from an pre-approved email address can come through to them. Failing that, Setting up local mail client to download by IMAP or POP, and set all approved emails to enter the Inbox and all others to end up in a Bulk or Spam folder, will help them to avoid looking at the spam and maybe doing something foolish. There's a lot of stuff out there trying to prey on ageing adults, to get their savings, credit card numbers, etc.
Assess what they actually use a computer to do. Most end up using only email and browser most of the time, maybe a text editor and a photo album and program, some music downloads. If it's something they don't use or won't use, uninstall it from windows.
Make sure to set them on a Limited account, so they can't add any programs unless done by an Admin account. I've used this for my teen and saved myself many extra hours of grief I'd have had in cleaning her computer every half year or so. During those years nothing was able to completely crash or destroy her system where I couldn't recover it with minimal effort later.
If XP is all they know and going to W7 doesn't work for them, then by all means return to XP and put Avast on it, which should give most the protection they need still.