Is the low hum on the recordings or is it a "ground loop" hum? If you connect whatever external device you have connected to the PC, such as a stereo system output, RIAA preamp (you can't connect the turntable direct unless it has a built in RIAA preamplifier and most do not) with no record playing do you her the low hum? If you do its a ground loop hum and you need to correct that. Ground loop hums are common with guitar amps and AC powered effects and the "quick and dirty" way to fix it is to lift the AC power ground on one device.
One other condsideration, the interconnecting cable between the external device and the PC audio input. It must be a shielded audio cable. If you use a "speaker" cable it is unshielded and AC hum can be picked up.
On the other hand if you are connecting a turntable, that does not have a built in RIAA preamp, direct to the PC that is probably some (if not all) the problem. First the output from the turntable needle is too low for a PC input, second you need the RIAA preamp to recover the full fidelity of the recording.