I thought I already answered this. I will try again.
Windows XP is on the harddrive on the new computer. If you take that drive out and put the old drives in, it will no longer have Windows XP.
If you install the old drives in the new computer exactly as they were in the old computer, when you start the new computer, it will run Windows 98. It will run just like it did on the old computer.
If the monitor is different or other hardware is different, there may be some problems. Some of that may just be Windows asking for the driver.