Tibor,
Is the user an Administrator on the PC? Are you allowing Active-x control?
Is this the only PC having issues?
Also make sure that you have "RunOnce" registry configured.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce
After checking all the settings, please try to upgrade again.
Regds,
JK
Do rate helpful posts-
~Jatin