I found a way to use GeForce4 mx420 in Windows 7:
download the driver "93.71_forceware_winxp2k_english_whql.exe" from http://download.cnet.com/93-71-forceware-winxp2k-english-whql-exe/3000-2108_4-131071.html,
unpack it to a folder. Then, right click the setup.exe in the unpacked folder, select the "Compatibility" tab; check "Run this program in compatibility mode for:"; select "Windows XP (Service Pack 3)" in the pull down selection. Then, click "Apply", and "OK" to exit the property change. Now, run "setup.exe", you will be able to install the driver for your Windows 7. This works for my Win 7 PC - right now I am using it to post this answer.