I had some problems as well with my dual boot WinXP-Fedora and something similar happend to me. This worked for me (in this order):
1- Install WinXp on the FIRST partition. This partition must be smaller than 8 Gb.
2 - Install Fedora on other partitions
(the /boot partition should be installed in the first 8,5 Gb of the hard disk).
Important: During installation choose: Don't install Grub
(or any other boot loaders)
At this point both Win and Fedora are installed but you can only boot Win.
You can access your Linux partition through a bootable disk or using the first CD of FC3 installation. When "boot:" appears in the screen write:
boot: linux rescue
To be able to choose Win or Linux (dual boot) follow this:
http://www.geocities.com/epark/linux/grub-w2k-HOWTO.html