Your post is confusing. Do you mean the Inspiron has wireless but it doesn't work presently so should you buy a PCMCIA wireless adapter ?
Check out the literature which came with the router or download the manual from Netgear. Presumably the router has wireless security set up so you are being barred from connecting to it because you don't have the passphrase. The passphrase should be included in the literature provided by Comcast -- or see the manual for how to access the router's setup screens.