For friends (repair/help): free -well at least no cash, food accepted - got freshbaked brownies for my last one
For friends (hardware purchase/replacement): Cost of hardware
For others (repair/help): $10/hr
For others (hardware purchase/replacement): 10% markup on parts, $10 per hour
If it's not in my town, I'll charge something for mileage but haven't had to yet.
When selling full systems, I normally just include the cost of parts + 10% markup. Now if the customer wants something fancy like a full install and config of a W2K/98 dual boot, then I'll charge something for the extra time.