Open the boot.ini file in a text editor. If you can't find this, it's probably because you have system files hidden and you need to un-hide them first.
Copy the line that refers to XP and ends in fastdetect and paste it into your boot file, now add the switch;
at the end of that line, so the end of the line will look like this;
(note the space between fastdetect and the /SAFEBOOT, the colon next followed immediately by MINIMAL). Note; you should now have your original line ending in fastdetect and a new line ending in fastdetect /SAFEBOOT:MINIMAL Whilst you're in this file, edit the timeout= line to something sensible to give you time to think. This line equates to the time in seconds your boot screen will appear, so something like 20 or 30 seconds may be appropriate, "save as" boot.bak and exit.
You should also note that the text within the quotes "" is what appears on the boot screen, so you may wish to change "Windows XP" to Safe Mode or something.
You will now have two boot files, the original and the new. The reason for this, is if something has gone wrong, you can always change back to your original file.
Now rename your original file boot.old (this is the file you will reinstall if it all goes wrong) and rename your new file (boot.bak) to boot.ini
Shut down the machine and re-boot. You should now get a boot screen for the appropriate time you set on the timeout line to choose which boot-up option you require.
I am a little puzzled why you want to do this, as pressing F8 during boot-up will give you the same option.