Any kind of conversion the OS is capable of can be done without a restart on any inactive drive. The C drive cannot be converted because its running windows. If you have a D partition on the same hard drive you could convert that without restarting.

Alot of people, when they acquire a new drive (who convert to ntfs at the time) make the mistake of adding the drive without removing the first. When they are not familiar with jumpers and where to mount a slave or master drive, its a disaster waiting to happen. They will either format the wrong drive and loose all their data or install XP over the wrong drive and loose it that way.
When installing a new drive and at the same time installing a fresh XP (ntsc or not) on it, it is always a good idea to remove the old drive to ensure the data is safe.