when you installed xp it rewrote the mbr (master boot record)... your linux is still there, intact... if, by chance, you followed the recommendation when installing linux and made a boot floppy, you can get in that way, then go to the boot loader section in linux and reinstall it, I believe...
You can also find the floppy bootup .img file on the 1st Mandrake cd... browse to the dosutils folder (while running windows), then run rawwritewin... when that starts, browse to the images folder and choose the cdrom.img file, and write it to a floppy. You can then boot your computer from the floppy and get back into linux, then see what you can do about reinstalling the bootloader :-)
If all else fails, you can boot from the linux cd, then start to reinstall it... let it just get to the point where it installs the bootloader, then quit... not top notch professionalism, but it has worked for me in the past :-)
Good luck.