Issue with css in Windows, not on Mac Options

[Wunk]

I have quite a strange issue that seems OS related, rather then browser rendering related.

Site: www.nibbits.com

On OSX this renders perfectly. But in Windows a div with a background/backdrop (noticable in the header, on top of the dark green backdrop, a slightly lighter hued one should appear) is missing and one of the logo's jumps around when moving over. Same goes for when clicking on one of the logo's to go to the subdomain of one of the sites, one of the background divs is missing here..

In Windows I see this both in Firefox and IE
On OSX it's error-free, no issues in Safari, Firefox and Chrome..

I aquired this site last december, so I didn't do the design or CSS programming, but I cannot figure out why Windows renders it differently from OSX

Anyone has a clue on what causes this problem ?