Help! 404 all over the place :(

Please help, I’m having a complete nightmare.
My site just gets 404 errors – http://files.red313.co.uk/blog
I’ve tried understandding the htaccess thing but it really is a complete and utter mystery.
I’m using the comicpress theme. It was working fine but then…it broke when (i think) I was trying to set my home page as a blank page and then have a ‘blog’ link in my menubar go to my blog.
Any help would be really appreciated. Again, I’m completely confused by the htaccess and permalink thing.
I’ve tried changing the file permissions on the htaccess to 777 but wordpress still doesn’t seem to be able to write to it (if indeed, thats what it needs to do!)
Again, thanks in advance for your help.