Apparently they did not address this issue in all these years, as I am using SW 2013 SP3 with windows 7 64 bit and sometimes I miss one or more open files.
When I press the "H" key I get all open files, so luckily there is a workaround.
Unfortunately i do not have SolidWorks 2011 installed to test, but i do not believe it is a generic software issue. For me it works in 2012 SP5 and 2013 SP3. My first things to try would be to check you running a supported graphics card, with the correct driver (check using SolidWorks Rx) and a windows Aero theme on the System. Running a non aero theme causes some graphics acceleration to be turned off in win 7.
I cannot find any suggestions or specific SPR's on the SolidWorks knoweledgebase at the moment. Perhaps there is a particular workflow that causes the previews not to be displayed is there anything in particular they were doing at the time, workign siwth some combination fo parts, sketches, aseemblies, drawings etc?