Problem with Win 10, old ASUS Radeon HD 4350 and ASUS VH236H monitor - picture is not fullscreen

Like the title says:
* windows 10 Pro x64
* ASUS Radeon HD 4350 EAH4350 SILENT/DI/512MD2(LP) 512MB 64-Bit DDR2 PCI Express 2.0 x16 CrossFireX Support Low Profile Ready Video
* ASUS LCD Monitor - VH236H 23" Full HD w/

Ant the picture in windows is not fullscreen for some reason. The resolution is ok: 1080p. But there picture is a blurry mess with almost 1 inch black bars around on all sides.

Feeling stupid... This never happened to me before. Need to confess - this is my first half an hour windows 10. Just trying out on an old rig, before considering upgrading my main rig to win10. Also, this is my second ever non-nvidia card in my life (the first being Voodoo 3 ages ago) :laughing: So I'm all around like a fish out of water...
The card is so old, there are no official drivers for it no more, it seems... All I could find was an annoucement that it was left in "legacy" support, and please upgrade to a better card :smile:

I this the problem with win 10, or that old card? Can this be sorted out?

Any help very much appreciated.