If Windows NT is on drive C:\, this may create a problem. If you can install WinXP on drive C:\, and WinNT on maybe drive D:\, then WinXP would detect that WinNT is there, and it should create a dual boot menu at for startup. I have to assume that you are using the NTFS format for your hard drive? I believe that WinXP should be able to handle WinNT files without any problems. You probably could get along by using only WinXP. However, if I remember correctly, WinNT is only capable of handling the max of 2GB? This is why it would be better to have both operating systems on different separate drives.
ONEderer