Well, these days you can also use boot0hfs instead of boot0 installed to mbr, which will look for a hfs partition instead of an Active one. That will allow you to keep windows partition Active, which in the case of XP it's not that useful (i think?!).
Or you can use another partitioning tool to mark the partition active... GParted live is usually the main suspect.
Terminal and "fdisk" are also a choice, from an OS X installation or install dvd.
Last but not the least, Disk Utility likes to mark as Active the partitions that it creates or changes file system in certain circumstances; try formatting the partition as fat and back to hfs, install Chameleon again and try it...
most probably it will do the trick