Website Fix Malicious Software

I need someone to fix this problem ASAP. Basically my site got attacked and malicious software was installed on some files. So when people get on the site it automatically causes some tracking software to be installed on the person's computer. I need someone to fix this and do something to prevent this from happening again. Here is the error message I got from google webmaster: Of the 36 pages we tested on the site over the past 90 days, 6 page(s) resulted in malicious software being downloaded and installed without user consent. The last time Google visited this site was on 2009-02-11, and the last time suspicious content was found on this site was on 2009-02-11. Malicious software includes 2 scripting exploit(s). Successful infection resulted in an average of 2 new processes on the target machine. Malicious software is hosted on 2 domain(s), including [login to view URL], [login to view URL] This site was hosted on 2 network(s) including AS26496 (PAH), AS15169 (GOOGLE).