You have to boot up you system with either a Windows Live CD or a Linux Live CD and repair your boot.ini file manually, if this is (hopefully) the only problem and no other files are corrupt.
A boot.ini file normally looks like this.
You may have to change the disk or partition number, depending on the hard drive, where your windows lives.
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptIn
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Let us know, if the problem is solved !!!