Have you tried waiting until GRUB loads, select your Windows XP partition,
then start hitting f8?

During boot up, after selecting Windows in grub, press f8 immediately
after hitting enter. You can select safe mode there.

Two issues here, one being the bootloader. Using f8 can be very hit
and miss at the best of times. One techie on the net suggested that
you interrupt the XP boot, then reboot. XP should then give you the
option to boot into safe mode etc as a matter of course.

When I bring up GRUB, there are two entries for Vista. The first is
a normal Vista boot, and the second boots Vista in SAFE mode. I
don't know how the second entry got there, but there may be something
you can add to your GRUB menu to add a SAFE boot entry for XP.

You can go into Vista and change all the entries and even their
sequence. My Vista is not open now but I will send detail tomorrow.