Requirements

OS: Windows 2000 / XP / 2003 / Vista / 7 (currently only the 32-bit versions are supported)
Memory: at least 128 MB (192 MB or more is recommended)
Note that you should have enough memory left for Windows (at least 128 MB for Windows 2000 / XP / 2003; at least 512 MB for Windows Vista / 7)!
Hard disk space: 2.5 GB (XFCE version) / 4.5 GB (KDE version)
Note that you need an NTFS file system (which is default since Windows 2000) because you can't create files larger than 2 GB on FAT(32) file systems!
A good internet connection (to be able to install further applications via apt-get / Synaptic)
Some basic Linux skills to proceed once andLinux is installed

What you will get

You will get:

a fully functional Linux system, however without the usual desktop (you've already got one from Windows)
a second panel (e.g. at the top of your Windows desktop) or a second start menu (in the system tray next to the clock), from which you can start Linux applications
Linux applications and Windows applications can be used simultaneously and you can cut and paste text between them
apt / Synaptic to install further applications

You will NOT get:

another desktop
the bench of applications that usually ship with Linux distributions (you have to fetch whatever you want)
trouble with further drivers ;-)

Security warning: It is recommended to use andLinux only on single-user-PCs or in a trustworthy environment because the communication with the X-Server and the launcher is not secured, i.e., every user who can login to Windows can access andLinux.
andLinux is not suitable for high-performance realtime graphics such as required by most 3D games. Although some users managed to get openGL applications to work on recent hardware, you will most probably experience problems doing so.
If you have a multi-core CPU, you will only be able to use one of these cores for andLinux (due to technological limitations, as all andLinux processes are encapsulated in one Windows process).