The first thing I would do is check http://sitecheck.sucuri.net/scanner/ and verify this is true.
If so, the best thing to do is assume your files are infected, and remove them. I tend to lean towards the scorched earth methodolgy, and just delete all the core WP files: http://halfelf.org/2012/scorched-earth-security/
Make a backup of everything and get started with removing anything that could be infected. When you've restored things, remember to change all your passwords. Finally, run a scan on your PC to make sure that's not the problem.