I do a clean install of windows 7 about once a year to get rid off rubbish that slowly builds up on my PC’s over time.
The problem comes in that after installing Windows 7 SP1 and possibly Internet Explorer 10 or 11, there are still over 200 patches pending which is over a 1GB of data. Add in the patches for Office 2010/2013 and that is a minimum of another 600MB.
Installing all these patches takes time and multiple reboots. And there is a bug with Windows update that causes a slow install of patches and spiking the CPU at 100%. You can read more about the problem on Microsoft’s own website – Microsoft KB 3102810.
Then install Internet Explorer 10 or Internet Exporer 11
and then run Windows update.
This could easily be resolved if Microsoft release a Windows 7 SP2 but this is not going to happen as they are concentrating on Windows 10.