strange thing I had the same issue today
I tried to repair the disk through the hitting R the first time as I was told by winXP. Went with chkdsk /R to repair all damaged files etc. This didn't cause it to work though. I couldn't boot at all from the disk. I had to repair the windows installation by hitting R a few screens later on the installation cd. After this repair install was done everything worked as it should and my profiles etc were all as they were before.