I had a number of issues after upgrading to osx el Capitan.
1) Paralells 10 immediately stopped working.
2) Booting to windows 7 didn't help either, Screen brightness was stuck at 100%, Mac keyboard functions did not work
and sound didn't work either.
Being hasty I decided to use the opportunity to upgrade to windows 10 which did not resolve any issues.
3) Upgraded to Paralells 11 trial version and everything works but system extremely slow. except problems still persist when using boot camp and booting into windows, (sound, screen brightness and keyboard issues)
4) Searched online again then found this link
https://support.apple.com/en-za/HT204923
Followed instructions exactly as per the link above and now everything is working perfectly.
Looking back I wasted almost a week trying to resolve issues when really all I had to do was carry out the instructions above which helped me sort
out all my issues in under an hour.
One other issue I was having was that I battled with downloading the windows support drivers, the download kept timing out and the deleting
or formatting process wouldn't work.
I discovered that the 16gb flash drive I was using was faulty only after using another flash ( 8GB ) drive when
I mislaid the 16bg one.