How to remove backslashes that appears and don't allow images or html elements display?

I have a problem that is perhaps a little unusual in our environment. When I edit a page of my site containing a link or image and then edit, and save, the following happens:
The image or the link does not appear!
I return to the page and realize that backslashes were added in all tags.

thus:

Edit the following tags:

After saving the page, looks like this (note the backslash (\) canceling the tag):

Clearly, these tags will not render!
And d@etail: this is happening on every page of the site!

Some solutions I tried:
1. At first thought to be the theme (used the Touch). Replaced by another and error continued.
2. Once thought to be the browser (use Firefox 10) I went to Chrome and the same thing happening.
3. I reviewed the entire CSS file for errors and nothing.
4. Finally I thought that could only be Drupal, perhaps with some problem in the core. CMS uninstalled completely and reinstalled again. I set up the site and to my surprise, it happened all over again.

I do not know what else to do, the site is stopped and really need your help.
Can you give me some indication of what might be happening with these tags?

Thanks in advance