I have the same router, WPA, 63 bit and no problems in setting it up or keeping it running. I'm wired, but I share a modem with a wireless, and that router does the job well right out of the box, and making a key. Try using either channel or 6 or 11.
I have a linksys router WRT54G, I have a buddy who uses linksys router. He as well as my self had problems connecting wirelessly until he changed from a busy channel. Upgrade the router to the latest firmware.
Any particular model Linksys router? What other equipment do you have connected? (cable modem?) Were you able to access the internet via the cable modem previously without the new Linksys wireless router?
Since it's brand new, this would be a great time to do a grand reset. Power up, wait until it stabilizes, and shove a paper clip into the reset button hole. Depress and hole for 60+ seconds without letting go. It probably take less time than 60 seconds, but I like to play it safe.
After it's reset, plug your computah directly into the LAN ports of the router. Forget about wireless for now. Go direct. Can you get to the web based setup screens?
Got what working? Connected to the router web page? Connected to the internet? Working via wired or wireless?
Sure, I do it all the time. However, you need to stop assuming that we can read minds and disclose some details. What works now, what doesn't work now, and what do you have to work with? The usual problem is trying to make everything work at the same time. That usually screws up. Get the cable modem working. Register your modem with Comcast using their signup page:
use the supplied Comcast cdrom. It works. After you get the modem running and registered, only then start playing with the wireless router.