Blog and Admin page not loading

About a week ago I started to get malware warnings when trying to login on my admin page and when loading my blog itself.

I deleted the users which had cropped up and changed passwords etc following advice from the wordpress forums.

However, I now can’t access the blog itself or the administration page. I get a HTTP 500 error. How do I get to my admin page so I can get rid of any malware if I can’t even load the admin page?

Any help greatly appreciated!