ad hoc wifi is tricky, I used the symbian s60 version and could get all kinds of devices to connect but not my lenovo x60 running windows xp.

I have also purchased joikuspot for n900 and I now can connect said computer without issues. I have noticed though that problems arrise if the 3g coverage is not good, I mean that joikuspot seems to not always work at all where the n900 browser works only slowly.

Try updating drivers (ofcourse) and try using Windows' native wireless manager instead of your adapter's propriatory manager, or vice versa.