Your post is void of information about the rest of the puzzling items. Such the make/model/cards/drivers/firmware updates and more.
It's not uncommon for the situation but clues are needed to give direction to what to look at. In my case I don't use the comcast wizard, but just set it up as any other wifi connection to the Wireless Access Point.
Bob
On my second wireless laptop, i am having a problem with slow download speeds and cannot get the comcast setup wizard cd to run. The download speed is about 700KBPS. What would cause this computers download speed to be slower than my other two computers? OS is Windows XP sp1.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic