problem with an advertisment?

[RAll Opinions Equal]

There is an ad for "Fix you PC" that is on my screen, off in the corner, ALL THE TIME. It is flashing red; it is painful to my eyes And I fear it will trigger a seizure. How do I block this particular Ad? How do I contact Yahoo to tell them of this danger. I can't find a place within help that allows me to contact Yahoo. Please help ASAP. Thank you
Hi, I've gotten two answers. 1) about anti-virus software; I have two programs running because I do a lot of gaming. I have run them both, I don't have a virus.
2) I should have been more clear; the flashing ad shows up when I am in Yahoo. I can't avoid this site because my e-mail is there.

So, I'm back to asking, is there a way to delete/block a particular ad from my Yahoo pages?

Thank you
R.