Security Problem!

Hi, I am having a security problem on my wordpress site.

I used to have my uploads folder in 777 to be able to upload pictures on my blog. But some script get in somehow and my site is redirecting to a 404 page. I can see that in any folder of upload there is an .htaccess with this redirect wich I am not being able to delete, but will try it with my hosting provider.
How can I be sure that this 777 setting was the main problema? it could be any other script or code in my site?
If I can solve this, how can I configure this folder to be able to keep uploading pictures?

Thanks in advance!!