If I understand you correctly, you want to restore the original XP
partition to its original size. If The XP partition can be rebooted
successfully, then it should be pretty easy.

Boot your live Lubuntu disk and access GParted. You can delete the
partition(s) created by the failed Lubuntu install (I'm assuming there
are two, including the swap partition). Looking at my dual boot
configuration with GParted, I see a Windows partition (/dev/sda3) and an
extended partition containing my Ubuntu partitions (/dev/sda5 and
/dev/sda6). Both of thee are part of /dev/sda4, which is the extended
partition you want to remove.

First, delete the swap partition /dev/sda6 and then delete the Lubuntu
partition /dev/sda5. Now you should have an extended partition which is
all free space. You should be able to delete it now, leaving unallocated
(free) space to the right of your Windows partition.

Now you can resize (expand) the Windows partition to take up all the
free space. That should do it for you. An alternate would be to reformat
the extended partition as NTFS and this should provide you with a
partition that Windows can use (probably becomes your D: drive).