Don't worry about it dude.... a friend of mine had similar problem some time ago(last year) while downloading some movie...i think may be ur ISP is either charter or comcast...just make sure that you don't click the link or provide any info about you to the link that they provide in email..DON'T MAKE ANY PAYMENT!!...just igmore their mail...
they don't know who you are at the moment so there is no legal way of finding that it's you which means there is no chance of going after you...your isp will send a warning mail that's about it...there is no way to prove that it's you unless you pay the money that makes you guilty then they will keep black mailing you which means more tension...so just leave it as it is...don't download torrents for a while...i mean who doesn't download torrent..there are freaking millions out there...it's just sometimes you get unlucky cos you downloaded some torrent that they provide to ambush...peace out bro..