I have Windows XP 32bit with a couple of administration accounts but as this is windows, the 'Administrator' account is disabled when you create new accounts with administrator privileges.
The problem I get is that when I upgraded to the latest version, it uninstalled my working version then failed to add user to the 'boinc_admins' group (Administrator). But this user does not exist anymore!!
The computer administrators have their own users names.
Why has this been changed when my older version worked perfectly??

Further information: I have tried to re-install older version downloads I still have on my PC and they no-longer work either (6.12.34, 7.0.25, 7.0.28 and 7.0.64). This would appear to be an update to Microsoft Windows that has stopped all addition to Admin users to plug security breaches in Windows flaky operating system.

Have you tried running the Boinc Installer a 2nd time, and Select the 'Repair' option?

Claggy