My two cents: Booting from Win7pe CD, it scans well xp-32, xp-64, win7-32 as already before.
For the second (and third) scan the "Download" dialog appeared again. I thought that latest data are already in the PE and said "No download".
Result: Error about missing uninfector database.
Running in PE you should not delete anything after scan. Propably within some minutes nothing will change, and on next start of the PE Uninfector.defs is by design not present.