Yes, I did change to pci express in the bios before fitting the graphics card. Of course on this pc, I had forgotten the password and had to deal with that first. Now with W10 running fine, I can't get into the bios as Windows starts before the usb. I will have to dig out an ancient ps2 keyboard.
The motherboard has the old type of AMI bios so the trick of getting in via windows wont work. I only wanted to change the boot sequence back from dvd to hdd. I cant find my old keyboard but can borrow one tomorrow.
It can't be emphasized enough -- BEFORE BEGINNING BACKUP YOUR CURRENT SYSTEM. Others have posted this too - but it can't be posted enough --so many people fail to do this and end up with no working computer that takes days to get fixed again.
It will take you at most 30 mins -- probably not even that long. Backup with something like FREE MACRIUM.
This will be very useful if you need to go back to W7 again or re-install W10.
If it is enabled, you will be able to use the USB devices during the boot. Hopefully you will not face the problem to boot into the BIOS using a USB keyboard/mouse once again.
The W10 is performing well, I cloned the hdd before doing the clean install and also used a spare hdd for the W10. I have two lots of the original W7 if I need to go back. As for the enabling of "legacy USB support", I always enable it on all my pc's. However in this case, it may not be enabled as I forgot the bios password and reset it. Legacy USB support may not be a default setting.
Using the ps2 keyboard, I got into the bios at the second attempt only to find that legacy USB support is enabled, still, I did change the boot sequence. It's unlikely that I will need to get into the bios again. There is always the trick of disconnecting the hdd to fall back on if a ps2 keyboard is unavailable.
I had similar issue and forgot what I did - try googling it for you specific mobo or machine