IDL 7.0 was release prior to Windows 7, so it would not have been tested on that platform and is technically unsupported (but may work).
Here is a link to a help article that details the platform and feature support for IDL 7.0:
http://www.exelisvis.com/Support/Help...
IDL was officially tested and supported on Windows 7 with IDL version 8.0.
http://www.exelisvis.com/Support/Help...