This is, I believe, a known driver issue with some nvidia cards and certain driver versions.
Here is one reference to it. I have had some very strange results with older nvidia cards, new drivers, and BF1942 or BF Vietnam. For instance, in several machines that I regularly use, which all have FX5200 cards in, any driver later than the microsoft validated one from october last year cause a hard lock when the game is run. Older drivers don't crash, but give texture errors.
Have you tried using microsoft update and installing the reference driver, if it suggests one?
Failing that, I would strongly suggest either getting a graphics card from a reputable manufacturer such as ATI
or a later nvidia card, IE FX5700 or later. They seem to work fairly well.
My shuttle has a 128MB radeon 9700 in it still, and it works fine with anything I've tried, including Doom3 at reasonable resolutions. For BF vietnam in particular, the most important performance option is gobs of ram, at least 1GB. BF1942 isn't as critical, but still benefits noticeably.
pca