Pretty much anything can be installed alongside with windows. It's just a question of how. Before you insall a second OS, make sure you either already have a second partition available to install it onto, or if not, defragment your drive and then create one.

If you're installing another Windows OS, you can just install it over the existing one, if you're not too picky about how the boot process works. The Windows setup program will automatically install a bootloader so you can boot each operating system.

If you're installing a non-Windows OS, it's a lot more difficult. Same thing as above, have another partition available, or create one. Most Linux distributions nowadays will handle the bootloader for you and automatically configure it. But make sure Windows is installed first, otherwise installing Windows will overwrite your bootloader and your other OS will be unavailable.

I'll list a few resources here that really helped me out.

Understanding Multibooting by Dan Goodel

Win95 + WinNT + Linux multiboot using LILO mini-HOWTO by Renzo Zanelli, courtesy of the Linux Documentation Project

How Do I Partition My Drive For Linux Use? by James Andrews, courtesy of LinuxPlanet

And right here at AO, you can see this thread by cgkanchi.

If you were a little more specific, people might be able to help you out a bit better.