In my father's case, I think it's that "good enough" tools became available for Windows systems starting in the mid to late 1990s (he was evaluating NT on Intel starting with version 3.1), and either stopped being available on Sun machines or became too expensive compared to similar software on Windows/Intel machines.
Getting back to the original intent of this thread:
If I'm working for a business, for myself, I give one run through my standard cleanup procedure, which usually takes about 15 minutes (Spybot + Hijack This, basically), and then I re-image.
If I'm working at my trainer job, I am working on someone's home PC. In this case, I have a different mandate. Technically, the agreement to get me to do tech work goes like this: They "pay" my company for three (or in extreme cases, six) hours of trainer time out of their Career Training funds ($250 or $500 - they get $3000 a year). Theoretically, the people doing this can sit with me and I will explain what I'm doing as I do it. In practice, people drop shit off and I get to it when I get to it. Most jobs are simple spyware cleanups or Norton/Mcafee removals, but I can use that money to replace parts if I have to, and it's not uncommon for me to have to buy a PSU or hard disk to complete a job.
Needless to say, that level of payment justifies more effort on my part. I've been known to spend several days fully removing spyware, just to see how it works, but most typically, if I spend more than about 90 minutes on something, it's wipe and reload time. Those people probably get something closer to the value of their money.
Generally the only people who get charged $500 are the real assholes or people who need major hardware overhauls (motherboard/RAM/CPU jobs).
I will not even get in to how my company justifies my doing PC repair work out of training funds but it winds up being a good thing for all parties involved. The folks I do it for get the work done "free" and my company makes a ton of money for doing it.