Most easy way to reset sitecore/admin password to b is just run following script on core database.
UPDATE [aspnet_Membership] SET Password='8dC23rEIsvuttG3Np1L4hJmJAOA=', PasswordSalt='joeLPwcwMq6L7kyuVfVS7g==' WHERE UserId IN (SELECT UserId FROM [aspnet_Users] WHERE UserName = 'sitecore\Admin')
If your admin user name is different then Admin just update the “sitecore\Admin” in the above script with your admin user name.
Good read on this topic - http://www.sitecoredevelopment.com/MarkGraber/2010/June/Cannot-Login-to-Sitecore.aspx
Thanks and cheers