I've been looking into this myself lately, as I'm getting tired of the in-built perpetual bloat of Win7. First of all, there are key retrievers that will scan the registry and get your encrypted windows install key back. Since you own it, there's no issue with getting the key, and MS even has official downloads for Windows ISOs.
But Windows is still Windows...
I'm rusty on the myriad LINUX distros, as I haven't used the OS since I built this machine (4 or 5 years?), but as a starter, I'd highly recommend Mint or any Ubuntu-based setup (make sure not to get the Debian version unless you know why you might need it). Those seem to be the most windows-like and easily accessible options. Once you get your feet wet, it's pretty easy to recompile the kernel the way you like it, or to get another distro on a dual-boot setup.
I use Firefox as well, for the plugin support, but you could also check out opera or something else. Software removal in Linux is as simple as typing "apt remove opera" in the console. Also, look into amaroK for audio playing, vlc media player for video needs, and every distro should come with OpenOffice. The only warning I'd have is, if you're big into gaming, WINE can have some issues with some games. For specifics, check
Here.
My main suggestion is, before you commit to something, don't be afraid to download the DVD/CD/flash image and boot from it to see what it's like before you install it to the HDD. I'm assuming this is pretty standard now, but if something specifically offers a Live!DVD iso, that's the bootable image.
I have some experience with Fedora, Debian, Gentoo, and SuSE, but I wouldn't recommend them for a starter setup. Let me know if there's anything else I forgot.