Help - Recovering from major hack

Hi,

I am hoping someone can help me restore - or upgrade to latest version, the Atahualpa theme.

After a good few hours wrestling with a very annoying hack/virus as per AVG screen grab attached here, an Exploit Script Injection, I seem to have finally stopped it by replacing the Wordpress files - which you could see extra PHP code at the top of the infected files, and also as the theme seemed to be actually kick-starting the hijack, I've had to replace the code via FTP from the original install, but now it's very obviously not working quite right and I can't get into the 'admin log-in dashboard' to update etc!

If I were to get in and auto-update the theme, do i lose everything, as I don't really understand this system? I mean, the lightboxes, pictures, posts etc? I gather the posts are on the server database, so they shouldn't be touched, but the lightboxes, header images??

If someone can help me sort this out, I'm happy to donate. I oddly enough can/do build websites for loads of people but am new to Wordpress, this site being set-up about a year ago. The attacks started a few months back, but updating the plug-ins and Wordpress always seemed to right things until now. You can see the website as it currently is here: www.marinecourtapartment.co.uk
I've attached the log-in screen as it appears to me and the screen beyond it if you input your details and try to log-in.

I'm now very keen to get rid of this stupid hack and get the site functioning cleanly again and hopefully avoid this happening again, so all help very very much appreciated. Plus, any advice on how to avoid it happening again.

Thanks
Chris

p.s. i didn't really describe what was happening: If you could get onto the site, you could see by the status bar the a website 'dudecars' or something was madly trying to do something, but more often, you couldn't get to the site, or log-in screen, you'd be taken to a site pretending to be a windows folder that was 'scanning' for virus problems... may the bastards who wreck peoples hard work rot!