Yes, it is possible.
How is the system set up ?
Is it set with a boot loader that gives him the option of which operating system he would like to boot into ?
Is the main drive partitioned with Windows 98 and Windows 2000 on separate partitions, or was Windows 2000 installed as a parallel install alongside Windows 98 on the same drive and partition ?
If you can give some more detail one of us will walk you through some options.