Just bought 2x8800GTS 640MB video cards. Having trouble with the way the drivers behave

If I use them in SLi, every game I play gets like a strobing effect. If I dont use SLi then I get no strobing effect.
Driver Issue makes no difference 158.22, 158.21? 162.15 all have the same effect. And yes I have been using Driver Cleaner Pro
to completely delete the registry entries too.
Would the way that XP was installed make any difference? (cos I was using Vista Ultimate before).

Motherboard - Albatron K8 SLi
RAM             - 4x512MB DDR400
Video Cards  - TwinTech 2x8800GTS 640mb

Didnt spend AUD$1200 on 2 video cards to have 1 sitting idle in my case