I'm assuming your left with XP since you didn't specify. In case you're not familiar with the boot.ini file, it is on your C: drive and hidden. You will need to get into folder options to allow hidden files to be seen. After you do that and are able to access the boot.ini file it should look like this when you are done editing it.
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(0)\WINDOW S
[operating systems]
multi(0)disk(0)rdisk(0)partition(0)\WINDOWS="Micro soft Windows XP Professional" /fastdetect
This assumes you are using WindowsXP, and it is installed on the first partition of your hard drive.