I see the same thing in my Laptop. I use a custom skin in XP, and removing Windows Search, did correct the Tabs appearance and the background of any QTTabBar toolbar. Re-installing Windows Search broke it again, so it's Windows Search is gone for now. (by broke meaning ANY skin used in XP QTTabBar toolbars are a plain color) Also, under options, Appearance, 'Use Tab Image' and 'Toolbar background image' are greyed out.
Funny thing is, I use the same Windows theme, QTTabBar AND Windows Search 4.0 on my workstation, and the TABs and backgrounds are fine. Also, under options, Appearance, 'Use Tab Image' and 'Toolbar background image' are Available.
Both machines are fully updated x86 XP SP3, all .Net Frameworks up to 3.5 SP1, same theme applied after using a uxtheme patcher. Not sure what the difference is. Anyone?