If your Dell has 10/100 Mbps network adapter installed, that is not a "wireless" connection.
What you need to get depends on what you are trying to do:
- simple file and printer sharing?
- internet connection sharing:
-- ADSL broadband?
-- Cable broadband?
-- Dialup connection?
In its simplest form you can set up an "Ad Hoc" wireless network by installing a wireless network adapter in each PC. From here, you can then enable "Internet Connection Sharing" on the internet connection of the PC that connects to the modem. This does, of course, require that the 'host' PC is running before the 'client' PC can connect to the internet. For Internet Connection Sharing, the 'host' PC must be running Windows 98SE or later, it isn't supported in first edition 98 or 95.
Next step up, if you want the PCs to be able to access the internet independently (without relying on a 'host' being switched on), would be to use a router and, if you want to be "wireless", you need a Wireless Router. However, if the router is located beside one of the PCs, that PC could be connected by ethernet cable with only the remote PC being "wireless".