Right Click on My Computer-> Goto Advanced->Under Startup and recovery click Setting->Under System Startup Click Edit-> A text file named boot.ini will open
In that text file you will see something like this:
Code:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect
under or above this their will be similar lines for Vista.
Delete that whole line, save and exit.
__________
That link given by Vimal explains it better.