Site Compromised

My site is coming up in Google searches as possibly compromised. I’ve informed my host, Godaddy, but strangely, Google’s own services via their Webmaster Tools don’t show a problem.

While I’m waiting for my host to look into things properly and Google to reconsider the site, I’d better try and take a look manually. But I have no idea where to start! Might there be any plug-ins that can check for altered code, etc.?

Help much appreciated; I sell via my site and this is particularly bad timing.