Double check your pc make sure it doesnt have onbard lan, as mentioned, a PCI card will cost you about a fiver, this will probably be based on the realtek 8139 chipset. Not as easy to "fit" as a usb device but would definately be my choice if the physical location of the pc in question)
I had a look at the link posted above, the spec of that router seems decent, that said, it may (will) have features that you will never use such as VLAn and VPN (I'm dubious actually about the performance of these features on a device in this price range but I have never used one so wont comment)
Disabling the SSID is a good idea, all your machines are running XP so I would use at least 128 bit WPA - deinately not WEP. Mac address filtering is also a good idea in some cases, I wouldnt have said it necessary in your case but it wont harm.
You also need to disable WAN access to the router (probably disabled by default) and put a password on it.
As you can see, you already have 2 similar but not 100% identical suggestions, there are lots of ways to do things, some right, some right-ish and some totally wrong, get your equipment, and have a go, then post back if you either get stuck or want more advice.
When setting up initially, use a pc with a hard wired connection, then setup the wireless without security initially and one laptop, then the next, and so on.
CALV