Linux is a fantastic OS and is far supirior to windows AND osX, otherwise known as mac. It also offers many installation options such as DWM(desktop window manager), Distro(everything EXCEPT the kernel), and installation type.
1. Choose your Distro
A debian derivative is fine here, I use (L)(X)Ubuntu. Just no KDE, and GNOME is risky. I recomend LXDE or XFCE as your session manager.
Section 2-Old PC
Damn Small linux, then Xubuntu, then Lubuntu at the best
Debian/ubuntu derivative. Maybe you can get away with KDE....
Section 4-Desktop/Gaming laptop(e.x. Alienware M1x)
2. Choose your install method
Only availible for ubuntu derivatives and linux mint, this installs linux as a dual boot. The unique thing is that linux is simply a program in windows, however for all instents and purposes is a conventional linux dual boot. Windows recomended, linux possible
down and dirty dualboot on a seperate partition
Clrean install. No other os's, and only one partition.
Section 4-live CD/USB
Installation onto a USB drive. everytime you boot up, your previous programs and settings are gone.
I recomend Wubi for windows dualbooting or if you have enough disk space to afford keeping Windows. A couple of tweaks are needed to get it working with Windows 8(see previous post) but they are easy, quick, and simple. See part 2 for installation instructions.