I did just update the plugins on Windows 7 too, and it worked for me.
Do you use any proxy ? You can configure them in Window > Preferences > General > Network Connection
Otherwise, try to reload the update site :
1 - Go to Help > Install New Software...
2 - Click on the hyperlink “Available Software Sites”
3 - Select the System Workbench repository then push on Reload
4 - Try to update again.
Hope it works,