www.linuxnewbie.org ? The name sounds right, though I’ve never really read the site. Check it out.
To answer your other questions to the best of my abilities (which are very limited);
You annot simply switch from one operating system to the other from inside one (if that’s what you meant). To get from one to the other, you have to reboot your machine. I run Debian Linux on one partition, and Win98 on the other. There is a program called Lilo (and there are other ones too) which is a boot manager, which gives you a list of your operating systems, and you can choose which one you want to use each time you boot. Or, like me, you can set one as default, and you have to master the art of pressing shift at the right moment to get the list to show up, or just have a 20 second delay with the list, etc.
You could install Linux on your D drive, as long as its bootable, I think. Though since this is your first attempt, I think you might as well just use your old computer for Linux until you know it a bit more. Alternatively, if you just want to see what the Linux environment it, you could try Knoppix, which is a version of Debian Linux with the KDE environment, though Gnome and other ones are on it too. You can burn Knoppix to a CD, make your CD-Rom bootable, and it will load a full distro from the CD. You dont have to repartition your hard drive, or anything. The only catch is that you can’t write to your hard drive, but you can use floppies/zip/other CD-ROM, etc.
If you do use your old box, you could either wipe the hard drive, or repartition. 2 gig isn’t all that much, though, so unless theres something you really want to keep (and you cant transfer it to your other box), I personally would just wipe it clean.
I haven’t used Opera, but I don’t believe it comes with the download. IIRC, Netscape is sort of the "default"browser, but it sucks. You can just download whatever you want normally, though, once linux is up and running on your box. I use Mozilla (and I use it in windows too) and its a great browser, IMHO.
As for what distribution of Linux to use - you might want to try RedHat. Generally, its the most user-friendly and easy to install, and there is a LOT of support out there for it. I’ve head good things about Mandrake too, but I don’t know it. Debian is harder to install, and requires more hands-on work to get it going, but as a user only (my SO does the insall/maintenance), I find it pretty easy to use once you stop thining like a Micro$oft clone.
Ummm…thats about all I have to say. Be prepared for some frustration at first, but once its up and running, its really stable and shouldnt require too much tweaking. Check out the RedHat and Debian webpages, read some How-To’s, and have fun!
Oh, one more thing - the advantage of setting it up on your old box once you have the install CD for RedHat (if thats what you choose) is that you still have internet on your newer one if something messes up! You still have access to help! Also, theres less risk to messing up the partitioning on your new computer.