Club 3D GeForce 6600 GT artifacts

Hi.

I hope someone can help me with an issue I have with my GF 6600 GT.
If in advanced display settings HW acceleration is set to "Full", I see black dots all over my desktop, randomly appearing and disappearing.
When I set HW acceleration to a lower value everything's fine. I don't experience any other problems such as game crashes.
I tried turning AGP Fastwrite off (through RivaTuner) , I tried different drivers (currently nVIDIA Forceware 81.98), different resolutions and refreshment rates and it's always the same - if HW acc. is set to Full I see black dots.
I went to the shop where I bought it, they gave me a new one but the problem remained.
Thx in advance.

Specs:
CPU: Intel Pentium 4 1.6 GHz
MOBO: Abit AI7 (5 PCI, 1 AGP, 4 DDR DIMM, Audio, LAN, IEEE-1394)
Chipset: Intel Springdale i865PE
RAM: 1536 MB (DDR SDRAM)
BIOS Type Award (02/25/04
Monitor : Samsung SyncMaster 753s
Graphic card: NVIDIA GeForce 6600 GT (128 MB) with 81.98 ForceWare drivers
PSU: 400 W
DirectX 9.0c
AGP 8x
Disk Drive Maxtor 6B160P0 (160 GB, 7200 RPM, Ultra-ATA/133)
Disk Drive ST380011A (80 GB, 7200 RPM, Ultra-ATA/100)
Optical Drive DVDRW DRW-3S167
Optical Drive SAMSUNG CD-ROM SC-152L (52x CD-ROM)