upgrading memory on old HP pavilion dv4000

My friend bought a used laptop last year that has been running very slow. She asked me to fix it and I found out she's running Windows 7 on an old HP pavilion dv4250us with only 512 MB of ram. Obviously it needs more memory to run Windows 7 smoothly. I'm not sure if it's worth trying to find memory for this old laptop or going back to Windows XP. She can't afford anything too expensive...