I went through this exercise a while ago. The MIC socket is most definitely NOT suitable for this job. The impedance and sensitivity are all wrong.
I bought a purpose-designed USB external box which can either take the 300mV line out from most systems or the non-corrected output direct from the turntable arm. This is the better option, if you've gat access to that signal, as it applies the appropriate correction to the signal from the cartridge.
It's worth doing for a limited number of albums. Bre901 is right about how the tracks get chopped up in an arbitary fashion unless you do some work on it afterwards. The software that came with by USB device is quite good at cleaning up the recording for scratches, pops, bangs etc but it is quite time-consuming. Google for the latest devices - I spent about £60 but you can spend hundreds, depending upon what you want to do.
I found it also gobbled up hard drive space. A 120Gb drive might not be too big!
I did some of Mrs Odd One's 1980's vinyl stuff a while back and she's now got it on her iPod. How we've made progress in the past 25 years!
Cheers,
the Odd One