I'm using Windows Xp Media Center, 1 GB Ram, CPU - Intel(R) Pentium(R) D CPU 2.80GHz, 2793 MHz, graphic card - integrated (Aida says this: Intel(R) 82945G Express Chipset Family), but you know what... this is irrelevant...
I just wanted to write that I haven't experimented with any kind of theming programs, when I realised that I DID change my default windows theme. Granted, I just turned off all those colorful xp elements, but it still counts as "theme modyfing". So I changed back to default "blue" XP theme and.... the buttons reappeared. But wait! There's more: when I changed back again to MY theme settings.... the buttons didn't disappear.
So... yeah, thanks for hinting me what to do, I hope this post might help someone else.