managed mode works but not the ad-hoc mode !

Hello all,

I have two labtops HP Omnibook XE2 with a Wireless PCMCIA Card Trendnet 
TEW-421PC (configured with ndiswrapper).
Both of them run under a Mandriva Linux LE 2005.
I would like these two labtops to communicate in ad-hoc.
Here is my problem : the managed mode works whithout problem but not the 
ad-hoc mode !
In ad-hoc mode the wireless card is power on (the first light is on) but 
don't send any packets (the second light blinks never) !
        
However the interface wlan0 seems to be correctly configured :
   
    iwconfig
    lo no wireless extensions.
    
    wlan0 IEEE 802.11g ESSID:"aron" Nickname:"guillaume"
    Mode:Ad-Hoc Frequency:5.26GHz Cell: 00:00:00:00:00:00
    Bit Rate=0 kb/s        Tx-Power:10 dBm    Sensitivity=0/3
    RTS thr=2347 B        Fragment thr=2312 B
    Encryption key:88ED-D
    B0-FDE4-C95D-670B-B890-CD    Security mode:restricted
    Power Management:off
    Link quality:100    Signal level:0    Noise level:0
    Rx invalid nwid:0    Rx invalid crypt:0    Rx invalid frag:0
    Tx excessive retries:0    Invalid misc:0    Missed beacon:0
    
    site0    no wireless extensions.
    
A ping request results in "Destination Host unreachable" and a "iwlist 
wlan0 scan" lists only my cell.
    
I don't understand. Any ideas ?
    
Thank you very much
jl