Fatal error: Cannot redeclare register_sidebars (on 2.5.1)

I have some widgets on my blog and I used Sidebar Widgets to let them work.
Then I upgraded to wordpress and at some point I get the error:
Fatal error: Cannot redeclare register_sidebars() in /var/www/vhosts/fmf.it/httpdocs/blog/wp-content/plugins/widgets/widgets.php on line 22

Reading the forum I found a thread (https://www.ads-software.com/support/topic/139194) in which I learned that my version of wordpress, the 2.5.1, has a native widget support and that the solution to my error should be delete the Sidebar Widgets plugin.

I delete the file wingets.php from the directory /plugins/widgets/ and the blog still work.
Then, at some point, my widgets for del.icio.us and for Google Search where updated and the new 2.5 interface notified me and I choose to upgrade them automatically.
I wrote my ftp password and after some crunch I went back to the plugins page and there the surprise: the Sidebar Windgets plugin was there again and the widgets del.icio.us and Google Search weren’t updated.
If I try to enable the Sidebar Widgets plugin I get the error in the title.
Where I went wrong?