nope, not running a homegroup.
my computer is the only one which is win 7. there is a desktop on wired connection (which doesn't seem to have any effect whether it's on or off), and the flatmate's laptop, which is xp, and as soon as it's connected to the wireless network, my speed disappears (still shows on the taskbar as being connected full strength).
Other things i've tried, by the way, are: rolling back the wireless card driver
to an earlier version, uninstalling it completely and re-installing with the latest drivers
if i plug a wired connection in, it works fine, so it's definitely a wireless problem (though i know that covers a hugely broad spectrum).