Did you install a boot loader when you installed linux? (Lilo or Grub). If you didn't, you're going to have to use your distro's rescue system (usually the first install disk) to load the system, then play around with it until you get the bootloader set up.
On the other hand, if you did, you might have set your bootloader to default to the windows installation. In "lilo's" case, hold shift down when your computer boots to get a list of OS's you can boot. I'm not sure with grub, but it's probably similar.