Malware Infected WordPress Site

Hello,

There is a website [url removed – Mark]. It is infected with malware, and whenever a PC visits the website or tries to login to the dashboard, it becomes infected with malware called Security Sphere 2012. If the website is searched on google porn advertisment appears. I believe a bad third party plugin was installed. Are there any suggestions or helpful websites I can visit that can guide me to eliminate the malware from the website and restore an appropriate google search? I have been searching for and answer with little sucess. I would also like to prevent this from happening again in the future. I am aware it is probably very neccessary to update to the newest wordpress, but the original web developer is afraid updating the wordpress will affect the appearence of the current theme. Thank you very much.

Best Regards,

Kelly