Hi for the last time,
If your dad uses the free version of MailWasher, and is careful, he can view each email without downloading from his ISP and he can delete any dodgy ones.
A regards Firefox Portable, with noscript and Web of Trust add-ons, there is nothing to lose and all to gain. There is nothing to learn and it is extra to IE6 not instead of it, except when Avast support ends do not continue using IE6. He could also, with both IE6 and Firefox use Dr.Web AV to check before downloading (e.g. any PDF document or Program).
My machine is 12 years old. My computer habits are cautious and as it has happened before, I am as concerned with drive failure as I am with infections.
Just clone/backup or be prepared to reinstall 98SE and all programs, without data, from scratch.
I really wish you both well but the choices will only reduce. Avast is currently a superb solution; but though Avast provides, I guesstimate, say, between 95% to 98% protection for 98SE that may mean being open to a possible 1,000 or so new infections every month.
My regards
PS Earlier this year I purchased a 250 GB Buffalo Ministation for less than £50.