I have installed several instances of OpenERP 6.1 on Windows 7 Professional Service Pack 1.
I know it needs an NTFS file system to work, so that would rule out XP with a FAT or FAT32 file system.
While I have never installed OpenERP on Windows XP, a search of the forums and documentation suggests that it is possible, so long as XP was configured with NTFS.
There are some comments (make sure 'show comments' is selected at the top of the page) about experiences with Windows XP at http://doc.openerp.com/v6.0/fr/install/ ... ation.html
Hope that helps!
Ray CarnesUrsa Information SystemsOpenERP Ready Partner