Ok I've seen that the MX6421 (and probably a bunch of other Gateways) are notorious for having this problem. Well, NONE of the solutions I've read have worked, but I finally found a manual workaround for this issue, and here it is:
■ In the control panel, "Add Hardware".
■ Find the uninstalled wireless network card (probably just says "Network Adapter" in the list).
■ In the Hardware Update Wizard, select "No, not this time" for Windows Update; press Next.
■ "Install from a list or specific location"; press Next.
■ Select the bottom option: "Don't search..."; press Next.
■ Select the Network Adapter option again; press Next.
■ On this screen, press "Broadcom" under the list of manufacturers and then select the following driver: "Broadcom 802.11b/g WLAN" dated 3/23/2006. Press Next and then "Yes" to the prompt. It should then install the driver for you and begin functioning like it should.
Note: I did all this AFTER attempting the install of several other drivers, including the one provided by Gateway on their website:
http://assets.gateway.com/support/drivers/search.asp?param=mx6421&st=kw It's the BCM40100.exe driver. So, me being able to select this driver in the end was as a direct result of me attempting the install of these drivers. But if you're like me and exhausted all other options, you probably already tried to install the driver.
I sincerely hope this works for you and you don't have to go through all the same trouble as I did. Best of luck to you.