I've found the best way to "burn in" a PC is to use it! I believe that burn in used to be more important in the PC infancy period as there were more unreliable products out there. I don't think there is a huge benefit today. The only reason I would burn one in is if I built for someone, just to be sure it was right.
DFI Infinity 975X/G, Intel C2D E6600 (@3.4Ghz), 2 Gb DDR2 800 GSkill HZ, Powercolor X1900XT, 74 Gb Raptor SATA, 250 Gb Seagate SATA, Audigy 2 ZS, FSP Epsilon 600 watt PSU, NEC 3540 DVD-RW, ASUS DVD ROM, Thermalright SI-128, Thermalright HR-05, Lian Li PC65 case, Samsung 940B 19" LCD