It is difficult to know for certain, but the spider you have described sounds like a female writing spider, also know as the black and yellow spider.
Writing spiders are found throughout the USA and Mexico, in southern parts of Canada, and have been founds as far south as Costa Rica. As you have discovered, writing spiders are quite distinctive, with their white bodies, orange and black legs and black head. If your female writing spider was the size of a half dollar, that puts her as about average, as they usually grow to around one-and-a-fifth inch in diameter. Male writing spiders are smaller and less colourful, mostly brown in hue.
There Is No Need to Panic!
Writing spiders are not poisonous, although they will bite if agitated. Therefore, there is no need to set about removing it from your home. Their diet consists of flying insects, and a female writing spider can take down an insect twice her size. Writing spiders also have distinctive webs, around two foot across, circular and of a dense, zig-zag construction. Tall vegetation or the underside of window frames seem to be its preferred spots to make its home. Here's a film of a writing spider capturing its prey: