|.I dont want to make a clean installation..|
You have no choice but to do a clean install. You cannot do an in-place install of an earlier version of Windows.
However, you do not need to delete all partitions, just the one containing Windows 7. If there is any data on this partition copy it to one of the other other partitions before you start.
Delete that partition and any recover partition that Windows & may have created. this is usually a 100 Mb partition right at the beginning of the disk. Create a new partition and install Windows XP.
You will have to reinstall all applications regardless of which partition they are stored one.