well, those diags can tell you if you have a bad CPU (or more likely that the CPU is overheating and faulting, meaning that the cooling is not up to the task) or if you have a bad memory module. (memtest86) if you find some fault, then if you bought the PC from an integrator then you should be able to file some kind of warranty claim on the whole machine, or if you built it yourself you could then buy new memory and return the bad memory. if the cooling is improper and you built the machine yourself then i guess you'd have to pay for a bigger cooler.
so no it's not the case that you would just have to run out and buy another computer...
rob