security concern with bridge mode in xp

i would like to hook up my laptop to my network but don't have an access
point. i have a usb 802.11b adapter that i plan on connecting to my desktop
(running xp pro) and bridgeing the connection. my concern is with security.
is there any way to enable some sort of security or will any machine in range
of the desktop now be able to connect to my network?

my laptop is a mac running os x, if that makes a difference.

any help would be greatly appreciated!
-nick