Old Games
Many old games, particularly those that were designed for MS-DOS, may not run properly under modern systems. This post describes several ways to get these games running.
Windows - compatibility mode
Windows XP includes a Compatibility Mode for running older Windows games in XP. In some cases, it even works with DOS games if you set the mode for Windows 95 or 98 compatibility. Microsoft.com has a page dedicated to this feature:
Run Older Programs on Windows XP
Emulators
DOSBox is a popular tool to emulate DOS functionality under Windows, Linux, Mac OS X and other operating systems.
DOSEMU is another one for Linux. These emulators should get old DOS games working under just about any modern operating system.
www.emulators.com has various Mac OS 7/8 and Atari emulators for Windows and DOS.
PearPC is a PowerPC emulator for various systems - essentially, this could allow you to
run Mac OS X (PPC) on an x86 (Intel/AMD) PC. For running Linux applications on other systems without resorting to full virtual machines or a proper Linux install, check out
LINE or
Cooperative Linux.
Booting
Often, the best way to get a program to run is to do so in its native environment, or as close to it as possible. This usually involves installing another operating system on the harddrive alongside your current one (eg. on another partition/drive), but can also be done by booting from a CD, floppy or USB drive. Most versions of Windows come with the ability to format a floppy disk with a DOS bootup.
Instructions here.
FreeDOS is an alternative. For Linux applications, you'll need to download an image of your Linux distribution of choice and install it, probably from CD/DVD - though some distros, such as
Damn Small Linux, are small enough to boot from a USB drive. There are some Linux distros that can run from a floppy disk, but these aren't usually suitable for Linux games.
Mac users have the advantage here - an Intel-based Mac can run Mac OS X and Windows XP natively, as well as Linux and DOS from boot. Mac OS X for Intel is not designed to run natively on an x86 Windows PC, but it is possible with some unsupported hacks. This setup is not always stable, depending on the specific hardware used; it's also not legal, since it involves modifying OS X.
Liero
A quick note on Liero - it will work fine in Windows xP without extra effort, but only with sound disabled. Running it with the "/n" command line switch disables sound, allowing it to run. Most downloads of the game come with a batch file (WINXP.BAT) which will do this for you.