Boot Manager is compressed
If by accident having compressed the entire boot partition, then Vista will display the following error:

BOOTMGR is compressed. Press Ctrl+Alt+Del to restart

Check points for fixing compressed boot-manager:

* Boot into Windows Recovery Environment (WinRE) and perform a “Startup Repair”.

* If the above doesn’t solve the problem then boot into WinRE again, but when selecting the Vista-installation, then       press “Load Drivers” (Instead of “Next”) and use the mini Windows Explorer to right-click the boot-drive and deselect       “Compress this Drive” and choose Apply to sub-folders and files.

* If that also fails then boot into WinRE again, and go to the “Command Prompt” and execute these commands:
o compact /U C:\bootmgr
o compact /U C:\ntldr

if you cannot find the compact command go to x:\windows\system32\ and execute the command.