Hi,
I had this problem too on my site, and your site looks the same as mine on my pc. Most of the background transparency layer is missing, and when you scroll the site it makes a real mess.
It happens on some pc’s but not all. For instance if i look at the joomlart uvite demo on my high spec vista laptop then I see it not working. On my lower spec xp desktop pc also running latest ie8 it looks correct. Looks the same on firefox on both machines.
Maybe it’s vista related, although I’ve tried it on another vista machine and it looked ok too so I don’t know.
Anyway, try changing the size of these files:
images/trans-bg.png
images/trans-bg1.png
images/trans-bg2.png
or images/colour/trans-bg.png if you’re using a different colour scheme.
They are 1x1pixel at the moment. Make them bigger, that worked for me.
I made mine 4x4pixels, just resize them in photoshop and see if that sorts it for you.