It may be that the backup program did its' own formatting of the drive. Since there is not a one size fits all format for interoperability between windows and macs.
Windows likes NTFS (mac's not so much, read only), Windows and MACs can use FAT32 (but MACs seem to experience file corruption using this format). If your external disk is either of these two formats (not very likely) then it may be as simple as opening the disk management utility in Windows 7 and assigning a drive letter.
But it's more likely HFS, HFS+ or HFSx and you're going to need something like MacDrive.
EDIT: Note: Every available disk that can be used to store backups is listed. If you’ve partitioned a disk, the available partitions are listed. Time Machine can’t back up to an external disk that's connected to an AirPort Extreme, or to an iPod, iDisk, or a disk formatted for Microsoft Windows (NTFS or FAT format). If you select an NTFS or FAT-formatted disk, Time Machine prompts you to reformat the disk. Choose a different disk or reformat the disk in Mac OS Extended (Journaled) format. Because reformatting erases any files on the disk, only do this if you no longer need the files or if you have copies of them on a different disk.
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Additionally if these "backed up" files and folders are in some type of proprietary archive format, used exclusively by Time Machine, it may be difficult to extract them for use on your Windows PC. If you still have the MAC, it may be better either to hook up the external back to the MAC and drag and drop the files onto the external drive, rather than having to worry about extracting them from the backup archive. Or and I wish I knew more about MAC networking, setting up a shared folder between the Windows 7 and the Mac and dropping the files that you need into that folder.
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