I came across MadMACs some time ago and it has become an invaluable part of my portable network security. Recently I've been looking to see what else is out there and found Mac MakeUp which provides a number of advanced features (integration with Wireshark, IP networking options, etc.) and has a straightforward graphical interface.

Both programs have been tested thoroughly and work a treat, but if you feel the only way to do a job properly is to do it yourself, then here is the step by step to DIY MAC spoofage:

1. Click Start->Run and enter 'regedt32' (no ' marks) to start up the registry editor.
2. Find the key:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\
Class\{4D36E972-E325-11CE-BFC1-08002bE10318}
   and you should have a list of 4 digit subkeys (0000, 0001, 0002 etc.)
3. Leaving the registry editor window open, go to Start->Run and enter 'cmd /k net config rdr' (no ' marks).
4. The console should list your PC and network details. Right click the start of the text next to NetBT_tcpip (the bit that looks like: {XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}) and right click->select 'Mark'. Drag over the whole text (as shown in the brackets previously) and then press CTRL+C to copy to the clipboard.
5. Go back to the registry editor and click on the first subkey (i.e. 0000).
6. Click menu Edit->Find (or press CTRL+F). Paste in the text from the console and click 'Find next'.
7. It should return a result very quickly (if not - you've done something wrong!).
8. Right click anywhere on the right hand side pane apart from on one of the listed items and choose 'New->String'. Give the value the name 'NetworkAddress' (case sensitive, no ' marks) then simply double click it and enter a new MAC address!
9. Reboot your machine and enjoy!

For more information, check this Wikipedia article on MAC addresses.