To answer your first question, the hard drive will simply be more active due to the much more frequent disk I/O activity from Windows. I'm not really sure what the root cause of the increased disk activity is, but consider that the memory requirements of Windows 7 are much higher than that of Ubuntu. Therefore Windows will be using the swapfile more often, and that will result in increased disk activity.
A second cause might be the numerous 'scans' that Windows runs. This includes the file indexer (unless you've turned it off), spyware / virus scans, etc. Those sometimes run without any prompting and can really put some stress on the disk.
The third reason could simply be the filesystem itself - Windows uses the NTFS filesystem by default and Ubuntu uses Ext4 by default. My own experience has demonstrated that Ext4 is much faster at bulk file operations than NTFS. (Nothing official, but certainly reproducible.)