Some good points are made here, it's odd, given the number of new computer users in the last 10 years & most of them has heard of backup, but percentage wise, fewer are backing up prior to lowered pricing that allowed most anyone to own one. And as stated previously, I know of few that has created recovery DVD sets. Even in the absence of a backup, the computer can be loaded factory new, software wise.
Recovery partitions are good & faster, but malware can corrupt these, making it unbootable.
And many consumers knows nothing about the importance of good security practices, including network security & keeping app apps updated to latest versions. Very few knows that it's best to use a tool such as Revo Uninstaller to remove Java prior to installing the new version & fewer than that doesn't even realize they don't need Java to begin with.
As Scoop8 has suggested, a good beginning would be an imaging service. There is a wide open market for this service & is highly under utilized, It takes those who knows how to perform the service to get many users to begin. It may take some time to get the service going good, but as I've stated above, word of mouth advertising is the best. Do a good job, leave the customer feeling happy & confident in your service, at first giving a card with 10% off for a referral, that person will tell others.
This service can easily be extended to virus/malware removal (the 100% free Emsisoft Emergency Kit is good for this) & app updating. Ideally, once the computer is free from infections, begin to distribute security solutions at some point, A one time scan & cleaning is a Band-Aid to the real issue, preventing infections by selling low cost solutions is the answer. If you're in the business, the more of a brand you buy, the less it costs. You need to stick with just a couple of mainstream brands, those that has less impact on the customers systems. That's one thing that I like about ESET NOD32 AV, quality protection & easy on resources. On my more powerful ones, I tend to go with Bitdefender.
Finally, learn all that you can about network security, as it's as important as that on the computer. Router settings can be easily hijacked though some of the out of the box settings. Including it's name as seen when broadcasting, some is not only the router's make, but model also. Like say "TrendNET652", that's all that a determined hacker needs to know to gain control of it, if all the owned done was setup a wi-fi passphrase & didn't glance at other settings. It takes only 5-10 minutes to correct the settings & rename the network, plus any time required to upgrade the firmware, which should be checked for & applied first, if the issue that's patched affects that customer. Some upgrades only are cosmetic & may add a feature or two that the customer doesn't need.
Or like updating the BIOS on a computer, may brick the router. So again, update firmware only if needed. Like, if there's a gaping security hole patched. Not to add a few advanced firewall settings.
These type of things would be better than to be prepared for any & everything & would make you a specialist in a particular area that is in great need. It's just a matter of getting a few initial customers to get the ball rolling. You may need to spend a little money on advertising to start, in time, this will take care of itself.
Back in the early 80's, I did essentially the same with a part time lawn business that grew so huge that I had to quit taking customers. But it gave me an advantage, I raised prices & could then choose the higher paying customers, dropping the $25-30 ones altogether. For about 10-11 years, I had 14 lawns to take care of (it could have been a lot more, but I needed my regular job for health benefits) & made good money. More than my job paid by far. Once my career changed to a different one, sales managment, I could no longer handle it, nor needed to, so I sold the truck, equipment & customer list to a good friend (I got with the customers for explaination & reassured them they were in good hands).
Today, that same friend now has 2 crews of 3 employees, performing all types of lawn services/landscaping that all started with what he obtained from me. It could have been that way for myself, but it was only to supplement my income, not make a career of lawn care. I never liked mowing to begin with.