Thank you for your feedback. DWF Viewer 7.0 is a very old version which was released before Windows 7 released.
I would suggest you download and try the latest version of Autodesk Design Review 2011, this verison is supported on Windows 7.
For your convenience, please get the download from: http://www.autodesk.com/designreview
Any further question please let me know.
PSEB-GE- ACRD PSEB