ioleo - sorry, i forgot to notify you ... we did the same: installed XP SP1, installed 1.03, uninstalled 1.03, installed 1.05 ... nothing shows up.
i'd recommend to update to SP2 and retry installation of 1.05, if this doesn't make the error message disappear i'ts highly probable your system is corrupt and you need to re-install windows from scratch.
christian
ps: i'd strictly recommend to setup any new system with older windows versions (withous servicepack or with SP1) behind a router or offline, otherwise if you work with a public IP address you'd might get a victim of worms within seconds ...
to be on the safe side you can download the
complete installation package for SP2 (266 MB) before and install it offline