There is no officiate support for Radeon 9000 for windows 7 from AMD site, and of course it mean they discontinue support for this old card. Most of the advance user prefer using mobility modder which can be found on driverheaven.net, it involve modding which changing several code in the inf ID's section. There is another way to install it and it is easier than modding method.
This tutorial base on Dell Inspiron 600m and Dell Latitude D600 which use ATI Mobility Radeon 9000. This method simply explain how to install windows XP driver in windows 7 using compatibility mode for windows XP SP3.
1. First you need to download your original windows XP drivers , for this tutorial, dell uses R82040.exe.
ATI Mobility Radeon 9000
Version 8.03_15973C, A05, 22MB
2. After that, double click to extract the file. (You can also extract it using winrar/7z etc.)
3. By default it will extract to C:\dell\drivers\R82040. Click OK and until extraction finish.
4. Now go to C:\dell\drivers\R82040 folder. Right click on setup.exe and select properties.
5. Select compatibility tab then, check the box "run this program in compatibility mode for" select windows XP SP3 from the list. Apply and OK.
6. Now install and reboot after installation finish.
Additional Notes :
It is better to right click and select run as admin to install instead of double click the file.