Boot Camp and Fusion are completely separate if you want them to be. Unless you want to use the boot camp partition under fusion that is. Fusion can boot using your Boot camp partition. I used it that way for a long time. I have since reinstalled XP into a Fusion disk image. Up to you which way you want to go.
If that's all you need, why not just use VirtualBox? It's free and works extremely well. It doesn't have the bells & whistles of Fusion, but it runs Windows perfectly. I've been using it with Windows XP for several months now with no problems at all.
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