Need a cheap way to get digital audio into computer.

[Pyrrho]

I've got an old computer that I plan to upgrade in the not too distant future, so I don't want to spend much money on it. However, I will not be replacing it immediately, so I would like to be able to use my old computer. I want to be able to record digital audio in my computer from an external source, but I don't have a digital audio input. I was thinking about something that can convert from a standard coax or optical (it doesn't matter which) digital cable to something that could enter the computer via a USB port. Does anyone here know of a very cheap way to do this? Thanks in advance.

(What I want to do is make a CDR copy of a music laserdisc that was never released on CD. I have a laserdisc player with optical and coaxial digital outputs. I use my computer for making CDRs, normally compilation discs for my car from CDs that I put in my computer, so I've never needed a digital audio input in my computer before.)