You can get music from many online stores, such as Napster, iTunes, HMV, Virgin . . . the list goes on.
If, as I suspect, you mean free music, this isn't the place to talk about it.
There are lots of sites devoted to free music from as yet unknown groups, but if you mean big groups, as I suspect you do, you'll have to turn to illegal means, such as file sharing, or legally ripping CDs.
File sharing is something we can't really discuss on this forum as it is illegal to share or download copyright material.
Frostwire will slow your laptop down because you've probably been downloading willy-nilly, and you're probably infected all over the place.
If you want your laptop to feel brand new, using Windows, a fresh install is the right thing to do, if you have a Windows CD.
If you don't have one, that option isn't available to you.
If you still want to use Windows, and have no problems, you'll just have to take more care of your laptop.
If you really want it to be quick, feel new, and have much less problems, seriously consider either a new laptop or a new OS.