Okay I'll take a shot what I can here.
The y adapter can work but only if you have a motherboard/video card intended to be used with one enables multiple display. Dell made some of these, the cables have a special pin out and requires some proprietary hardware. Otherwise you are out of luck to do it with a splitter type of cable.
This may be an older version of the digitus usb 2. to vga adapter. This website has drivers for the current version. It is possible but unlikely it could work for you. I'd tend to think if you had this device working in windows 7 that you should be able to get it going in Win8. Assuming you had 32 bit windows 7 and went to 32 bit win8(or 64 bit to 64 bit)
Here is a link for this device from Digitus there is a driver link on the right side of this page.
If this device driver doesnt work then you need to figure out who makes this usb to vga device for us to be able to help you with it. Either follow the instructions above to find the hardware ID in device manager, or find some identifying name on the device.
If as you said you have a video card with one connector you could consider replacing this card entirely with a card that has two ports. This is a pretty standard feature and a decent card won't cost you a lot.... You could also buy just a cheap card to do this and pull it/replace it with your current card for gaming. You'll need to determine what type of slot the desktop has and if you have some kind of slim case find a card with dimensions that will fit but not a real tough thing to do.
You could also buy a cheap card off ebay or such that fits your case dimensions and is of the right slot type to fit one of your spare slots.