how to install avast on the server scan for hacked files??

scan and remove of course ,because i get 2 alerts for troj

one from sucari sitecheck -----Details: http://sucuri.net/malware/entry/MW:SPAM:SEO
and the second one is reported by my client,who saw this alert from avast on his computer
can i remove them manually?how do i know where are the files on the server that been hacked?