Have you checked the manufacturer's website for updated drivers for your
graphics card, drivers specific to XP?
What screen resolution were you previously using before you upgraded, are
you using the same resolution now? A lower resolution will increase the
size of anything on the screen. Control Panel\Display\Settings tab, check
the slider on this tab, you may have the resolution set higher than in the
past or you may want to lower it.
The issue of not filling the entire Window is partly related to resolution
and the fact most websites are designed for a resolution of 800x600 while
many people are using 1280x1024 or something close to that. If you change
to 800x600 using the slider, before doing so, click the advanced button on
the Settings tab, on the General tab, from the DPI setting dropdown list,
choose normal size, then change the resolution. Things will get quite small
when you first change the dpi but when you change the resolution the will
become larger again.
If it's still too large, you can return to the dip setting and use custom to
set it a little smaller. Some pages may still not fill the browser Windows
but understand, this is more a function of how the pages were designed in
order to be more easily read across the spectrum of most PC monitors and
settings.