HI, it's been a while so I may have some details wrong and Josh has now bought a copy of XP anyway so it may not matter.....
When installing NT4.0 from the original CD (pre SP3, I believe), there is a limitation in the size of partitions. You can get around this by loading a new IDE/ATA driver from the step in setup where you're asked if you want to install any other/SCSI drives/controllers.
With that done, you should be able to create much larger partitions but I am unsure what the limitation is then.
Maybe I have this wrong and it is just an upgrade to fdisk for NT.
A google should give the details.
Another solution that might work (again, not sure!) is to try installing from the upgrade disc. At some point in the installation, you'll be asked to feed your NT or W2K disc into the drive to prove you qualify for the upgrade.
Groeten,
Ferdi