NT 4.0

Yes, u can, if....
U have your harddisk divided into 2 or more partitions
If not, you need to add a second harddisk
In both cases, you install NT 4.0 on your second harddisk ( D:).
Your system becomes a DUAL BOOT. (you will get after the completion of NT installation, a screen where you have to choose to boot with Win98 or NT...)
Be aware, if you decide to install NT on an NTFS harddisk, you will not be able to access the NT files from Win98 (which is FAT32). Though I recommend NTFS due the improved security. Password LogOn in Win9x is a joke.
If you didnt purchase NT 4.0 yet, I would recommend Win2000 Pro instead (= NT 5)