MBP late 2008 HDD and SSD setup
If you're simply wanting to dedicate your old HDD to Windows 7 duties you don't need to use Boot Camp to do it. Simply partition and format the HDD as FAT (Windows will insist on it being formatted as NTFS when installing), reboot your Mac while holding alt/option and insert your Windows install media. You will then be able to install Windows without further issues.

Boot Camp is only necessary when dual-booting from a single drive. OS X requires a GPT partition to boot from whereas Windows 7 expects an MBR partition. Boot Camp creates a hybrid partition table that works for both systems, but this process isn't necessary if each system has its own dedicated drive.