Even if you have GRUB installed in a seperate partition, you will need a third party boot loader to start it. This can be the Windows NT boot loader (NTLDR) or any other. There is a guide in the tips & tricks thread on doing this.
If you have two hard drives, you can install the Windows loader on the MBR of one, and GRUB on the second one. This way you can safely reinstall windows without over writing the grub loader.
I however have not tried/found the need for such a method, as it is pretty easy to restore GRUB/LILO after a windows reinstall.
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Hardware - Celeron 266 MHz, 128MB RAM, Intel i740 8MB RAM
Software - Slackware Linux 10.1; Archlinux 0.7; Windows 98 SE
Humanware - GS/CS C++ L+++ w b+++ DI+ D+ e
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