Did a quick test with another XP system, also this system could't connect. Seems the only system that can connect with my config is the windows 7 system.
Tonight I will do some wireless sniffing to see what's the difference between Windows 7 and XP connecting. Starting my OpenVPN tests now.
Can I suggest the following:
Install the crda package to get the country regulatory stuff actually applied.
For your configuration
config 'wifi-device' 'wlan0' option 'type' 'mac80211' option 'country' 'NL' option 'hwmode' '11bg' (or maybe better: option 'hwmode_11n' 'g') option 'channel' '5' option 'disabled' '0' config 'wifi-iface' option 'device' 'wlan0' option 'network' 'lan' option 'mode' 'ap' option 'ssid' 'XXXXXXX' option 'hidden' '0' option 'key' 'XXXXXXXX' (and make sure this HEXA only key has more than 8 digits)