One of the first few lines of /boot/grub/menu.lst will indicate which entry should be booted by default:
# Modified by YaST2. Last modification on Thu Aug 20 23:52:49 CDT 2009
However, if this /boot partition is something you want to get rid of, then you will need to replace the current MBR with the XP's MBR using the recovery console. This is done by booting to the Windows XP install disk.
The linux /boot partition may have a backup of the original MBR. Double check the filesize. If is is 512 bytes, it will also contain the partition table of the original mbr. The boot strap loader is in the first 440 bytes. Please read through this Wikipedia article on the MBR:
before doing anything. Also keep in mind that if Linux was installed previous to this last one, this backup_mbr may contain an old Linux MBR. You can restore an mbr with "dd if=<backup mbr filename> of=/dev/sda bs=440 count=1.
Be sure to use "fdisk -l" and "fdisk -lu" and save the results on a pendrive, or print them out. This will save the information on your partition tables. If your MBR partition tables become damaged, this will allow you to use fdisk from a live distro to rebuild the old values.
Doing this for every backup would be a good idea as well, so you are sure to backup any changes. You can also backup your MBR as well.
Also google for "NT Grub dual boot" for web pages on how to use the XP's loader to chainload Linux. This entails installing the grub MBR to a partition or disk with Linux, cutting it out to a file on your C:\ drive, and editing C:\BOOT.INI so there is a Windows boot menu with a Linux entry. This will leave a pristine MBR on your main boot drive. For Vista, there isn't a BOOT.INI file, but if you create one, Vista will use it.