It could be plugins, it could be your theme, or you could be hacked.
Longer answer follows. ;)
Visit http://sitecheck.sucuri.net/scanner/ and scan your site. It never hurts and if you come up as hacked then skip to the hacked links below.
Even if it comes up clean you still may be hacked but a positive from them would save time.
After that look at the usual troubleshooting 101 steps and try
- deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
- switching to the Twenty Eleven theme to rule out any theme-specific problems.
- resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
- re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.
After you've switched to Twenty Eleven and confirmed that your plugins have been deactivated then visit your site. Still getting that advertisement redirection?
Lastly, post via pastebin.com your
.htaccess file. I have a feeling that's where the hack is.
If after all that you now feel your site is hacked then proceed to look over these links.
You need to start working your way through these resources: