<p>Did Installing OneCare cause a "Generating user instances in SQL Server is disabled" error?</p>

<p>The only change that I've made to my computer is uninstalling AVG and installing the trial for Microsoft OneCare. Did OneCare change the SQLServer installation somehow?</p>

<p>This is a very "odd" question but is something I would post on EE in hopes of someone having had the same issue and giving their solution.</p>

I would look more at the uninstalling of AVG as the culprit.  OneCare does not care or even notice SQL Server instances as far as I can tell where as AVG does.
I would look into your SQL Server instance and check the jobs.  One or more may have been added by AVG.  You should remove them.  You might also want to drop the AVG database.  Just to be sure.
Note: I have never
uninstalled
AVG.  I just have notice some of what it did to my Database when my SysAdmin installed it.  Being an Accidental DBA I haven't had the time to properly evaluate it's actions.