What happened? All users suddenly have the username "admin" - Joomla! Forum - community, help and support

please advice? users in de jos_table have username:
admin.

has site been comprimised? i'm running 1.5.26 , virtuemart 1.1.8.

should delete registered users? , how can prevent in future?

please advice

welcome joomla forum!

how define "suddenly"?
did install extensions, or other admin related work (e.g. change 1 username yourself),
and afterwards usernames have been changed?

do have access apache server logs?

do use other 3rd party extensions?

your joomla latest release in 1.5.x sersies.
but virtuemart version old. there newer version available, security release...
upgrading (after creating back-up) recommended.