Security for network with WAP

This is probably a really newbie question, but...
I have a small office network with an 8-port wired Linksys router. Our office computers, and 2 network printers are on it.
In order to give our guests wireless access, I purchased a Linksys WAP, which I connected to the wired router. Setup was easy -- so far, so good.
The security on the wired side is where I need help.
We have file sharing on the office computers, we have network discovery turned on, etc. Between our computers, we share a lot of files and folders. I'M not about to try to password protect files on each of these computers. And since I'm behind a router, I do not use firewall software on these machines. (I check security with Home of Gibson Research Corporation and always get a 100% stealth rating)
So... How do I set up the WAP so guests can get to the internet, but make the "wired side" of my network invisible without a bunch of grief?