Hi, If you buy a wireless router, you can directly connect the desktop like now to it and laptop can go wireless or wired to the rounter like desktop. I am pretty sure your laptop has built-in Ethernet card as well in combination with wireless. So you can mix wired, wireless on wireless router to access Internet.
I am assuming that your desktop is connected directly to a DSL or cable modem via an ethernet cable. If that is correct, then I advise you to purchase a wireless router. Depending on your needs, a Linksys WRT54G should suffice. Feel free to choose a wireless router that suits your specific need(s). Anyway, after you purchase your wireless router, you can then connect your alleged DSL/Cable modem to its WAN port via ethernet cable. You can then attach your desktop to one of the 4 LAN ports via ethernet cable. You will be able to surf the internet as you did before but a little bit safer being that the wireless router (depending on the model you purchase) will act as a firewall. After you purchase your wireless ready laptop computer, you can initially use your desktop computer to communicate with the wireless router (read the manual for instructions) to set up its wireless feature (encryption, MAC Filtering, wireless mode, etc). If your laptop comes with Microsoft Vista or Linux Ubuntu, connecting wirelessly *should* be a snap. Anyway, I hope this information is useful. Take care.