Judland is definitely right about use of a second HDD being a risk-free way to handle this. I had no problems with a dual boot install myself, but it certainely was a bit frightening the first time I tried something new and risky.
You don't need to be able to code to use Linux. I do know how to code, and I know for a fact that I haven't done more than glance at some Linux and Linux application code out of curiousity. You may eventually learn how to compile a program, but fear not... it isn't difficult, and you don't need to look at any code or anything, and you DON'T need to compile anything if you don't want to. (Typically you'd only compile something if you want to use the most updated version of some software.)
The USB modem is something you should look up on the net. Linux works best with Ethernet, and many external modems can handle both ethernet and USB interfaces, so if you have an Ethernet interface on the modem and computer I would consider switching to it. (In this case Linux would have no problem interoperating, guaranteed). However, if that isn't an option, looking up the model of the Modem along with the word "Linux" and possibly "usb" on google is a good start for finding compatibility. There are some hardware compatibility databases out there for Linux (can anyone recall where they are?).
I'm not too sure about WebCams and Linux... I never used my WebCam even in Windows. Someone else will need to help there.
Finally, as far as your apps go, many will be replaced by Linux Native apps. I'd say that this counts as a fresh start in some ways, but that the total cost of the new programs is mostly the time to learn them. I did purchase a few programs, including Sun's StarOffice and Transgaming's WineX, although it turns out I use Abiword more than StarOffice.
If you list your favorite apps here we can list those that can't be replaced with Linux Native apps at the moment, although that list seems to grow shorter every year. Some Windows only apps can be run with Crossover Office (www.codeweavers.com) such as Quicken, MSOffice, Lotus Notes, and several others. Transgaming's WineX can run many games if you are interested in those (www.transgaming.com).
Judland made the whole switch eventually... I use my Windows drive to store music, movies, videos, documents, and other architecture independant files, but can still boot to it when I need to (...which has only happened to play some Morrowind).
Tell us more and we'll help where we can. 
obsidian_fox