Please make sure your system time and date are set correctly.  Also, if you have proxy information entered into Internet Explorer, Impulse may be trying to use that.  To make sure there are no proxy settings that could be interfering:

In XP, go to Control Panel -> Network and Internet Connections -> Internet Options -> Connections -> LAN Settings:

In Vista/7, go to Control Panel -> Network and Internet -> Internet Options -> Connections -> LAN Settings:

If all else fails, please contact support@stardock.com.