How do I do a clean install of XP?

Okay,

The best way to go about this, is to use a Windows 98 startup disk to delete all partitions from the drive.
Make sure to backup all critical files and data first.

Determine what size partitions you want, and what format to use for each.
(If you want to be able to access files in the XP partition from W98, then all partitions should be FAT32.)
Off the top of my head, I would do the following:
C: (W98) 8.3GB (FAT 32)
Remainder of drive will be the "extended DOS" partition)
D: (XP) 10 - 16 GB (FAT32, or allow XP to convert to ntsf)
E: (shared files) remainder (FAT32)

Install W98 onto the C: drive.
Get all drivers installed, and run Windows update.
Make sure that it runs fine.

Install XP onto the D: drive.
Select "Custom" and select dual-boot mode, and D: drive to instal XP to.
Choose to convert the volume to ntsf if desired.

You will likely have to call Microsoft, and explain why you need to re-activate XP.

Chas