What's the native resolution of your monitor, does your graphics card (and its driver) support this resolution and what is the setting in XP, because, from Display resolution:

One use of the term "display resolution" applies to fixed-pixel-array displays such as plasma display panels (PDP), liquid-crystal displays (LCD), Digital Light Processing (DLP) projectors, OLED displays, and similar technologies, and is simply the physical number of columns and rows of pixels creating the display (e.g. 1920 × 1080). A consequence of having a fixed-grid display is that, for multi-format video inputs, all displays need a "scaling engine" [..] to match the incoming picture format to the display.

[Emphasis by me.]

If your monitor has to scale, effects you describe can be the result.

What's the native resolution of your monitor, does your graphics card (and its driver) support this resolution and what is the screen resolution setting in XP, because, from Display resolution:

One use of the term "display resolution" applies to fixed-pixel-array displays such as plasma display panels (PDP), liquid-crystal displays (LCD), Digital Light Processing (DLP) projectors, OLED displays, and similar technologies, and is simply the physical number of columns and rows of pixels creating the display (e.g. 1920 × 1080). A consequence of having a fixed-grid display is that, for multi-format video inputs, all displays need a "scaling engine" [..] to match the incoming picture format to the display.

[Emphasis by me.]

If your monitor has to scale, effects you describe can be the result.