Virus on my wordpress

Today I had a warning when I went to my website from my antivirus that there was a virus on the website.

I checked with hostrocket asap and they fixed it. Here is what they said:

Hello

It looked like their was a virus attached to your index.php page and gallery2/main.php page at the bottom. I have removed these for you and urge to check to see if the scripts you are running are vulnerable to any known exploits.

What could I do to check and/or prevent this from happening again?

Thanks for any help.