Am I infected or something?

I’ve had to unlink my username from my site on wordpress.com because someone there said that everytime they open my link their norton detects a virus threat. Well I ran the exploiter scanner and indeed there was some virus hidden in the codes by the name Eval. I removed all those files. I was wondering how it got there? The only plugins I’ve recently added are googlesitemap, stats etc. I had to remove those.
So now that thank God my site is virus free, how can I protect it against any future malicious viruses trying to attack it?