Server Admin - Clean up Exploited Scripts and verify server

Hello, I am having some major issues with my dedicated server. It seems there has been some exploits on the mail system and also some of the programming on submit forms. This causes the website to lag horribly. See the site at [login to view URL] I have telnet access and plesk, and would like someone with experience in website security and server administration to advise me and preform any necessary updates needed to the system. Thanks, Lou