I assume your existing computers are on a router?

you will need a new wireless router..the router shud have
4 ports on the back which would act the same as your existing router
but the new router also has wireless so the laptop can connect that way
and the 2 desktops remain wired.

so no..you do not have to convert the 2 desktops to wireless

something like this would be what you want

Linksys model # WRT54G


"The Linksys Wireless-G Broadband Router is really three devices in one box. First, there's the Wireless Access Point, which lets you connect Wireless-G or Wireless-B devices to the network. There's also a built-in 4-port full-duplex 10/100 Switch to connect your wired-Ethernet devices. Connect four PCs directly, or daisy-chain out to more hubs and switches to create as big a network as you need. Finally, the Router function ties it all together and lets your whole network share a high-speed cable or DSL Internet connection, files, and other resources such as printers and hard disk storage space."