If you used an upgrade cd of Vista instead of a full version cd, the upgrade version subsumed(I never really liked that word and imo its a poor choice by definition) or replaces/nullifies the use of the prior version.
In simpler terms. Use of an upgrade version of Vista on the same machine replaces the license and ability to use the XP version on that same or another machine.
Options: Remove Vista, reinstall XP, return Vista(if within 45 days limit) for credit and if still interested in Vista purchase the full Vista version.
...winston
: Now that just sucks. I own both but I may not be able to use both?
: : > Since you used your XP as the basis for an upgrade, you no longer have a
: > license to run XP
: >
: > : >>I am running XP on my C drive and using Vista upgrade disk installed Vista
: >>on my D drive. I need to do a clean install of XP back on the C drive. Can
: >>I do this without messing up Vista? Thanks Carl
: >
: