Ran into this problem with a client of mine.

Symptom:

• Everytime the user tries to print a report from the web, the error message was shown and the report won't print. 
• The user is just a regular user and not assigned to Power Users / Local Administrators group.
  p.s. Elevating the user privilege by assigning it to Power Users  / Local Admin will solve the issue, but for audit purposes, this is not allowed at the particular client.
• Even after installing the necessary RSClientPrint ActiveX control as Local Administrators, the problem still persists.
• Tried applying the solution from this blog entry: http://blogs.msdn.com/b/mariae/archive/2010/04/13/error-unable-to-load-client-print-control-when-printing-reports-in-reporting-services-summary-of-all-possible-causes-at-april-2010.aspx and the problem still persists.

Platform:

• server: SQL Server 2008 Service Pack 2
• client: Windows XP x86

Causes:

• Insufficient user permission 
• Potential bug introduced in SQL Server 2008 SP 2?

Working Remedy:

• Deploying the RSClientPrint-x86.cab from SQL Server 2008 R2 to the client machine fixed the problem.

References:

• http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/799e7c86-1d2d-445f-9b18-687c1474e0a7
• http://stackoverflow.com/questions/340151/reportviewer-client-print-control-unable-to-load-client-print-control