when i try to install xp over ubuntu, a message "cannot mount volume" comes up, any advise
Sure, Windows XP doesn't know what to do when it sees a linux partition on the drive. Simplest way to fix this is to grab your ubuntu boot cd and proceed to the hard drive/partition setup point. From here delete the linux partitions and save the changes. Now reboot your computer without completing the rest of the Ubuntu setup and switch back to the xp cd.
I've done this many times myself.