Installing XP first, set up the XP boot files in an active partition. When you installed Win 7, it set up the Win 7 boot system and incorporated the XP files within it, but the Win 7 partition was not marked active. By deleting the partition, you deleted the boot system and are now back to just the XP boot after reinstalling XP. You probably could not delete the partition from Win 7, and now you know why it would not let you. 
 
IF YOU DO NOT WANT XP BACK, use disk managment from XP to set the Win 7 partition active. Then use the Win 7 install DVD to do a startup repair 2 or 3 times to restore the Win 7 boot. If the partitions are on different hard drives, you might even want to disconnect the XP drive.
 
You could do a startup repair...