A clean install is not a supported option for upgrade editions of Vista.
Some people have reported success with this procedure.
http://www.winsupersite.com/showcase/winvista_upgrade_clean.asp
Note that as this option is not supported there is always a chance that at
some future date a Windows Update or validation may fail because you used an
unsupported install option. If you do an in place upgrade from XP here are
some tips to help the process go smoothly.
1) Backup your PC.
2) Back it up again.
3) Test your backups.
If you skip the above steps please don't whine that the upgrade trashed all
your files. Any process that involves this many changes to the file system
is fraught with danger.
4) Run the latest version of the Upgrade Advisor and note anything it flags.
5) Uninstall (not disable) all antivirus, antispyware, firewall, disk
utility, and system utility programs even if the upgrade advisor doesn't
mention them. You will need to install Vista compatible versions after the
upgrade is finished.
6) Uninstall (not disable) all programs that the upgrade advisor flags as
possible problems. You will need to install Vista compatible versions after
the upgrade is finished.
7) If possible remove all hardware that the upgrade advisor flags as
incompatible.
8) Make sure you have Vista compatible drivers and software for all your
hardware devices burned to CD. Don't just look for drivers that the upgrade
advisor mentioned. If possible have drivers ready for everything.
9) Physically unplug any external devices like portable hard drives,
printers, card readers, flash drives, cameras, etc..
10) Run a chkdsk on all the partitions on all hard drives still connected.
11) Defrag the system and boot partitions.
12) Start the Vista upgrade process.