If the only OS I have installed is SuSE Linux, if I tried to install Windows, how would I stop it from deleting SuSE?

For starters, you would need seperate hard disk partitions for each OS.

Absolutely need a separate partition, FAT16, FAT32, or NTFS, depending on the windows version. You will also want that windows partition to start before the 2GB limit, meaning if you have a 3GB partion for Linux, and your Windows partition is after that, you're going to have trouble with windows. Best option is to partition your drive with Windows at the beginning of the drive and Linux at the end.

Another option, though is installing vmware, then you could run windows on a virtual drive inside Linux.
Still another choice is using Wine or Wine-based application, and run windows applications in Linux without the overhead of running a full windows operating system.

Here's what you will need:
a seperate partition on your hard disk and a boot disk (either your floppy or the installd CD1)

Installing Windows after you have installed Linux will end up with your mbr overwritten - this means you will lose grub or lilo. Once you've installed Windows, use your boot disk to return to Linux and then reinstall lilo or grub.