Under "System Properties" | "Advanced" tab | "Startup and Recovery" (Settings button) | "Edit" button...the Boot.ini looks like this when XP is the only available boot option...
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1) | WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1) | WINDOWS="Microsoft Windows XP Home (or Professional) Edition" /fastdetect
...I used the "Pipe"(|) character in the "boot loader" menu above instead of the "slash"(\) character, so it would be properly formatted.
When you see "Unidentified OS" or a second instance of the same OS/any other OS as a boot menu option, you'll need to delete the last line in the Boot.ini that starts with..."C:\Unidentified operating system on drive C." (or whatever happens to be the second OS choice in your particular case), save the file and reboot.
On a side note, you can also edit the Boot.ini from the Recovery Console using the bootcfg command.