Close friends and family i dont charge but anyone else its a flat fee of £50. This is for either building or fixing a pc and includes all labor costs right upto handing it back to them. This sorts the wheat from the chaff usually, anyone who is argueing about 20quid here and there isnt the sort of person i want calling me every week with another problem they have caused.
Quite frankly compared to the purple shirt brigade what i offer for 15quid less is ten times better.
Do not let yourself get into the habit of doing dead cheap work for friends of friends, word gets round and before you know it everyone wants stuff doing for cheaper and cheaper each time. You feel in a bad position because friend A knows you did a job for friend B and charged less etc etc.
Also be prepared to spot troublemakers, the type who will call you up at any and all blue screens 6 months down the line and assume you are their bonded slave because you charged them 20quid to reinstall windows.