There's a couple things you can do to fix this problem. First, you can try running /stand/sysinstall, editing the partition table, making the Windows partition bootable and then writing/quitting without making any other changes. You should then be prompted to install a boot manager; I'd pick the FreeBSD default.
If you just want to get Windows working, you might try booting from the XP CD and using the rescue console to fixmbr/fixboot.
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