Don’t NTFS Compress OS HD
Never compress your main hard drive (that has Windows installation in it, usually C:\).
It will modify the MBR and thus Windows will fail to boot. You will need to fix the MBR with the Recovery Console, accessed by booting from the Windows Setup CD.
You can copy your installation CD to a USB, using WinToFlash to make it bootable.
Once inside the recovery console, type “fixboot” and thus the MBR is fixed.
I read an alternative way to fix this by booting from a live CD/USB (Linux ok) and copying the three MBR files.