Connect laptop-desktop?

Is this a ng for this?

Have a Dell desktop 3rd floor with a Lincsys Cisco wireless router,
Comcast cable internet. New Winbook laptop 1st (usually) floor,
built-in 802.11. Both WinXP. Laptop connection works fine, but alone.

How can I get access to the desktop from the laptop? Access to
files,etc on upstairs machine. I tried and screwed up the wireless
connection, so I don't want to guess again.

Also: I have no problem with mail, as I use yahoo. But Connie uses
Eudora, which being on each machine, seems to get mail only to
whichever machine is used to download it. Then, as Comcast server
deletes on download, she can't get same mail on the other machine.
I've tried checking "leave mail on server"...hasn't worked yet. Ideas?

Thanks, Brad