when i do small jobs like that i charge per job not per hour. its generally 20-70 for most repairs and to build a computer it's under 100. the way i charge is whatever they can afford to pay, if i know they are rich or willing to pay more, i charge more. if they have limited funds, i charge less than normal. just use common sense
oops, forgot the most important detail: have a backup plan in case they are non-paying. either ask for half the money up front and the rest right before you finish or give back their computer. i am a bit diabolical as i have had MANY people pay me little or srew me completely. in those cases i install a revenge script on their computers, if all is well i deactivate it, if not the script will destroy everything on their computer at a time i set.