I work for a company that setup a website, this one thelinearshop.com (warning this site contains malware so don't go there if you are concerned), before I started working for them.  The site uses OSCommerce.  It now looks like someone was able to infect it with a malware link and jumble the site itself and when I visit the site I get a warning page from Google with the choice to leave, go to http://safebrowsing.clients.google.com/safebrowsing/diagnostic?client=Firefox&hl=en-US&site=http://thelinearshop.com/ for info on why it is getting blocked, or to ignore the issue.I have looked at the site and someone definitely hacked it.  I have never run into this before and I am not sure how to proceed in order to get this fixed.  I know I need to get the virus out but I am also concerned that whoever set the site up used an older version of OSCommerce and that it must not be very secure.  Is that the case?
How to fix a site that Google tells you is infected
security;oscommerce
Your best bet, if it is possible, is to set up a development version of their site and try to upgrade it to a newer version of OSCommerce and see if it works properly. I don't use OSCommerce but I would think they would offer upgrade scripts or something similar to help automate the process. Assuming it works properly I would then upgrade the live site to the new version. That way you are sure you have all of the latest patches and have closed whatever hole(s) were originally exploited.Once you've cleaned up the site create a Google Webmaster account for this website if you haven't already. In there you can request that Google check the site again and have them remove the unsafe website label from their listings.