CS Screen Size Problem

I am having big probs with CS. I had a Geforce 2 ti grafix card, athlon 1400, Epox 8kta3 mobo and things worked fine running at 1024 x 768. Then 1 day when I started game, the playing screen had been reduced to the top 60% of the left hand side, with me desktop showing on the rest. If I drop it back down to 640 x 480 then all is ok. If I use direct 3d then its ok also. I’ve since installed a Geforce 4 4200 but its the same problem. I have updated 4 in 1 drivers and latest detonator drivers. Seeing as it works in direct3d I was wondering if it could be open gl prob. Please help cos me cards wasted at 640 x 480.

BTW I’m running Windows XP Pro.