As far as I know, in Ad-Hoc mode the Windows driver force the usage of channel 3 (2.422 GHz) and doesn't use the ESSID at all (or it use to be that way).
So, could you try to set the Wavelan on this channel ?
Other things that may matter :
The firmware 6.06 and windows driver 6.14 are supposed to have "improved ad-hoc networking". That may be a source of incompatibilities, so you may want to downgrade your Windows and card setup to earlier version.
Some firmware release of the Wavelan don't like tcpdump.
Try Linux-Linux communications if you can, that may tell you were the trouble is.