I just upgraded my PC from Windows XP to Windows 7 and my pc does not start, not even in safe mode, I changed the boot to boot from the CD and then from the hard drive and the computer does not respond and work for hours with window start up. Now I can not even downgrade to XP as it does not run on the cd.
May be your computer does not have minimum requirement for using Windows 7. And now it is not possible to access with the previous version of windows Xp's cd.
Entire your Windows 7 bootable disc and start setup again. When it appears all the HDD partitions, delete the primary partition and eject your Windows 7 disc.
Then entire disc of Windows Xp and re-format deleted partition to complete setup Xp.
Minimum Requirements for Windows 7;
* 1 GHz of faster 32-bit or 64-bit processor
* 1 GB RAM for 32-bit or 2 GB RAM for 64-bit system
* 16 GB available HDD space for 32-bit system or 20 GB for 64-bit system
* DirectX 9 graphics device with WDDM 1.0 or higher driver
