I had the same problem on a Lenovo J115 when I changed from Vista to XP. I have been dicken' around for 2 days because guess what, Lenovo doesn't have drivers for my Viewsonic VA903b monitor. NOTHING I tried worked. Until just now!
I put in an old video card I had laying around that someone had given me a couple years ago, and PRESTO.
All is good again
I think I got the card idea from snooping around THIS site, thank you very much