XP will see Parallels and Fusion as two separate installs.
Not much you can do about it I'm afraid.
Don't confirm the install (can't think of the MS name right now), just run it as two separate installs for the 30 day trial period and pick the lucky winner.
You *will* be able to duplicate a Parallels and Fusion machine and run two or more machines concurrently. Not sure what the XP licence says about this.
BTW if you use the Fusion import tool from Parallels, XP sees this as a major change to the environment and gives you 3 days to validate the licence (not surprising as the virtual disc controller, virtual NIC, and just about any other virtual piece of hardware will change.