Writing basic web pages in html, write simple programs in JavaScript, C# programming, and word processing.
Anything faster than an Atom will do, including an AMD E-350 or E-450. Compiling C# will be the only thing you'll end up really waiting on, and for school, chances are you won't big enough projects to have to really worry over that.
Don't get Windows XP. 7 or 8 64-bit are the ways to go. Windows XP's updates stop next year, the newer versions are much more secure, utilize newer CPUs better, 64-bit-only games are on the horizon, and more RAM is nice to have and use.
HTPC: Home Theater PC
Here's a nice cheap starting point:
http://www.newegg.com/Product/Produc...82E16856115045
It only takes
2nd-gen Core CPUs (Fi, Core i3-
2120), and is tiny, but hey, $100 for PSU+mobo+case. $100-130, or less, for a CPU (Core i3--Pentiums would be fine, but the i3s are faster, so your call), $20 for an ODD, and you're in business.
P.S. With an i3-2120, and 24x Sammy burner, assuming you already have DDR3 RAM, and an HDD, I get a little under $260 shipped, starting from the above barebones. A video card would need to be low-profile, but with a $100 video card budget, that should be easy enough to do. Honestly, though, since you have a gaming PC, already, I'd use IGP, unless you wanted multiple monitors through digital outputs. Programming will not get you into the areas where SB IGP isn't good enough.