There's a subtlety I think I didn't make clear. The importance is in the disposition, not the action. You can always refuse service; a person should not get mad at another because they won't do them a favor (remember that next time someone won't do you a favor, btw). But if you help, don't treat the person like they're an ass for "making" you do it. A favor should show loyalty, not exasperation. Either say no, charge for the help, or make it an actual favor. Acting in a manner that inspires guilt for "making" you help them is just as immoral as actions designed to make you feel guilty for not helping. But seriously, pro tip, learn to take joy in helping others, that way when you do help you'll actually enjoy doing it. The beneficiary of your services will pick up on that and respond positively.
A noteable exception to this rule, btw, is when applied to a girl you're even slightly interested in; the right move is to, while channeling the complete confidence of Kirk, refuse to help her and, with a smile on your face, poke fun at her computer skills. If done right, she'll panic at the failure of her sexual prowess and will proceed to plead for your assistance to prove she still has control. She does not, you do. Refuse again, she'll persist, so explain, still while smiling and channeling Kirk, that you know girls love to get guys to do stuff for them for free and that you're not so
easily controlled. She'll probably act offended or more desperate for help; either way ignore her emotions then offer her a compromise: if she cooks you dinner, you'll fix her computer while she's cooking. Bring a semi-inexpensive bottle of wine, then without asking pour her a glass to drink while she cooks. Fix her computer, come back and sit to her right (or left, not across), eat dinner while teasing her that her porn browsing adiction will only lead to more problems. If this doesn't sound right to you, consider the following: you have shown yourself to be ultra alpha-male, you have put yourself in a position of control, you have a date with a girl you're interested in where she is submissively making you free dinner, you kept her busy so she didn't see how nerdy you look when you hack, and you've left her feeling grateful for it all.