Donít panic, XP is still there (unless you do something to mess it up). Your experience is fairly common and should be simple to fix.
The boot loader is probably having a problem finding the linux partition. As detpenguin pointed out, we need to know which distribution you installed. Some details about your hardware would be nice, such as whether you are using SATA drive(s) and how many hard drives are in use. Also, where you put XP and linux on the drive(s) would be helpful.
If the whole Grub thing is confusing, there is an online manual (http://www.gnu.org/software/grub/manual/grub.html
) that you can look through. It will explain some of the commands that someone will suggest you try after we hear back from you.