What you will need to do, is re-size your current hard disk's partition using the disk management program. Then, on the remaining hard drive space, create a primary partition, format it in whatever file system you want (fat or ntfs) and then what you will ned to do, is boot up from your desired operating system cd. Windows 98 / ME will only see the new partition and install fine, Windows 2000 will see both so be very careful.

I personally would go with Windows 98 or 98 SE on the 2nd partition

Other option, which IMO is much easier, would be to put in a 2nd hard drive, and install windows on that