kooshy: ndiswrapper should not be changing your MAC address. If you want to verify your MAC address, run the command 'ifconfig wlan0'. It will tell you the MAC assigned to the interface on the first line:
I suspect that the problem you're having isn't related to the MAC filtering, but to the fact that the network uses WEP. Are you positive that the Windows driver that you loaded into ndiswrapper supports WEP? Did you check the ndiswrapper database to see if other users have reported being able to use WEP with your card? If you're not sure, please let me know the PCI ID of your wireless card (which you can get using the 'lspci -nn' or 'lsusb' commands) and I'll try to look it up.
wlan0 Link encap:Ethernet HWaddr 00:19:21:85:0d:87
inet addr:192.168.1.47 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::219:21ff:fe85:d87/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:698425 errors:0 dropped:0 overruns:0 frame:0
TX packets:363146 errors:0 dropped:0 overruns:0 carrier:0
RX bytes:562341562 (562.3 MB) TX bytes:51003073 (51.0 MB)
Interrupt:20 Base address:0xe800
Also, make sure that when you enter the WEP key in Network Manager, you choose the right kind of security. There should be two options for WEP: '128-bit Passphrase' and '40/128-bit Key' (see attached screenshot). In some versions of Ubuntu, WEP passphrase is selected by default, but few people use that. You probably use a 40 or 128-bit HEX key. Try with that option.
If you still can't figure it out, if you post the output of the command 'sudo iwlist scan' and tell me the name of your network, I can give you instructions for trying to connect to it manually from the command line, which can be useful for troubleshooting.