well you need a converter, switch is free for 30 days, here- http://www.nch.com.au/switch/plus.html it's very good really, now most computers cant play flac unless they have the right "CODEC", which is a little file that unlocks a bigger one its very simple just download this codec pack, and install it, it will allow you to play many media types, including flac, and no it cant have any negative effect on your computer, so relax and enjoy cheers codec pack here- http://www.free-codecs.com/XP_Codec_Pack_download.htm
You can just use FLAC along with a GUI of your choice (I use FLAC Frontend) to decompress your FLAC files to WAV and then encode the WAV files to MP3.
I second the Easy Audio converter. It handles many audio types, which means you may use it again in the future. An all around good program.