Acronis is backup/restore software. How did you plan on upgrading from XP to Win7 with it? Even if you had a backup of a Win7 system there is no guarantee the coreect drivers would be installed if you were to restore to another computer.
I don't think it can be done or at least not easily. Why not just do a custom install of win7? It should just rename your old windows (XP) directory to windows.old so that you can still access any files if you need to.