Cool Edit Pro has an excellent $US30 plug in for converting to (and from) MP3 files & I think Cool Edit 2000 has something similar. But I wouldn't convert to MP3 unless I intend to leave it that way. There's a small (negligible???) quality reduction going to MP3, although you might find it acceptable if the original is an LP or tape.

Another option of course is to convert all your LPs to MP3 & burn to a CD ROM. You can get 10-12 hours of music on one CD ROM, & if you have a high quality sound system connected to your PC this makes a lot of sense. My PC is connected to my HI-FI, so I've done a lot of stuff this way. Make 2 copies so you have a backup. Otherwise it's a lot of music gone west if the CD ROM fails.

Cheers,
Alan