(HL2) Invisible objects and tentacle creatures

First: I am on a military network, so the sites I can get to are
limited by websense. Keep that in mind.

Sager 9860
Geforce Go 6800
Nvidia driver version 7.7.7.0

The tentacle creatures are invisible, so I pretty much hit every single
one, not knowing they are there.

Sometimes wooden walkways are invisble. I have to fire my handgun in
front of me to see bullet holes. This is inconvenient when Combine
soldiers and ant lions are after you.

Nova Prospekt: Plenty of invisible objects. I can see their shadow, and
that is the only way I know they are there. This is now officially a
big problem. I have found ways around the issues in other areas, but I
am at the room in Nova Prospekt where a carrier zombie suprises me.
There are two vents on the ceiling. I assume I am to stack objects high
enough to get up in the vent and climb to the other side. This is
somewhat difficult when all the objects in the room are invisble. I am
stuck, and can't go any further. (Unless my thinking is wrong on this,
and I am supposed to be somewhere else in the game).

I have also tried the current Omega drivers, and I get the same result.
Help? I really have searched Google prior to this, and I have found
nothing like this problem.

I don't have any problems in any of the other games I've been playing:
KOTOR, DOOM3, VampiThe Masquerade.

Any help would be appreciated.

Russ Chewning