First hard drive C:
Second hard drive D:
CD drive E:
I don't think XP creates a Virtual drive (like '98), but if it does, your CD drive will become F:
It all becomes a little irrelevant anyway, because if you boot straight from the CD drive (go into the BIOS and make it your first boot drive) XP during the installation process will ask you where to install. C: will always be C: no matter what, so you shouldn't overwrite your '98 installation. Just keep an eye on the installation drive letter. You wont of course be able to install to your CD drive letter, so if you mistakenly attempt that, you will get an error.