We need more information. Did Windows start to boot up, then fail? If so, you probably corrupted some system files and you best bet is either to try to find out what file(s) and either replacing them, or doing a system restore.
If Windows wasn't able to start booting, you could have messed up the MBR, or else messed up some specific boot files. If this is the case, you can try Super Grub Disk (Google It), to try to fix the MBR.
But, as mischief said, more specific error codes and more specific information (even if it may seem unimportant at first) would be helpful.
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