Well first of all make sure that the wireless is indeed turned on (don't worry, it is a very common mistake with laptops that have built-in wireless). If it is, can it detect any other networks?
Also, how is the desktop connected wirelessly? If it is through a USB dongle, plug it into the laptop and see if you can use that to pickup the wireless signal. that will confirm that it is a problem with the built-in wireless. Otherwise use a crossover cable form the laptop to the PC ad enable internet sharing. Can the laptop use the net fro the desktop's wireless?
When did this problem start? is it a new router? new laptop? has the laptop been able ot previously connect and all of a sudden stopped?