it is essential that you partition your harddrive,
I would do at least 2 partitions, one for the OS and one for the virtual memory file. if you install Windows 98 you can install Windows XP into a different partition and when you restart your computer you will get an option of which operating system to boot to, and then you can edit the boot-up options in Windows XP (from the Control Panel or My Computer go to System and in Advanced go to Startup and Recovery).
For a good partitioning program you can use Partition Magic, which you get at www.partitionmagic.com