I tend to construct an ad hoc among several laptops with QNX 621 and Windows Xp.

For a test, i used 2 laptops with Window Xp and QNX 621 respectively.

First, i configured the adhoc mode in Xp, it should be okay. Second, i tried to set the adhoc mode under QNX 621 in the 
following way:

#io-net -d orinoco channel=11,mode=adhoc,network=*** -p tcpip
#dhcp.client en0
#nicinfo
...
Total Packets Txd OK---------------2
Total Packets Txd Bad---------------0
Total Packets Rxd OK--------------4
Total Rx Errors-----------------------0
BSS Type--------Non-standard AdHoc
....


And the 2 laptops couldnt connet each other. 
Thx for specifying the reasons.