Windows VISTA and 7 require BCDEDIT for this. With XP (and NT and 2000), you can use any old text editor. First make sure the file is not marked Read Only. Then open and edit your boot.ini file. If you don't know where that is, you probably shouldn't be messing with it. :)
Add the second line, below, based on the values from the first line. Make any changes appropriate to your machine:
multi(0)disk(0)rdisk(0)partition(2)WINDOWS="Microsoft Windows XP Professional" /fastdetect
multi(0)disk(0)rdisk(0)partition(2)WINDOWS="Safe Mode" /safeboot:minimal /sos /bootlog
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