USB is to Firewire what VHS was to Beta...Firewire is faster but every PC built within the last decade or so has at leat one USB port.
Also, it's possible to (rather cheaply) add eSATA to your machine (assuming you don't have it already) if you're not afraid to open the case for a few minutes:
http://www.newegg.com/Product/Produc...-070-_-Product
There are more, this is just to show you they're out there. If the machine is recent (<2 years) it'll work and it's not too hard to install. The reason it might be worth it is because eSATA is smokin'-fast, about the same as if you installed the drive internally.