Honestly, with all of the junk/noise inside a pc, do you think that you can isolate the drive, power supply, d/a and more enough to really make it worth while. Why do you think that they make transports with seperate power supplies and DAC's. They do it to isolate each function to improve clarity and purity of signal.
I've ripped apart many a PC and can't imagine how much static, dust and noise from the cooling fan occur. Plus, with the PC hooked up to a phone line/dsl/printer/monitor it would simply make it that much harder to isolate audio without any interference.
Finally, with the drive and dac sharing the powersupply with everything else, you will have a hard time keeping the signal clean.