During the install you should make the spare partition(s) you're going to put linux on in ext2 or ext3 format.
If it all goes horribly wrong, and you are using Windows 98, you can boot the computer from a Windows boot floppy and type:
at the command prompt. This will wipe the boot record and should allow Windows to boot from the hard drive. If you're using an NT based version of Windows (2000, XP, etc), this may not work (I don't know about them so much) but I think you can boot from the install CD and perform a rescue operation to reinstall the Windows bootloader.
If you decide you want your linux partitions to be used for Windows again, you may need to use the Windows fdisk program to delete and recreate them, then re-format them.
Hoep that helps