Ditto for Toastman. I have a DFI lanparty and had the same problem, switched to Marvel - problem gone.
The funny thing is, I installed WinXP Pro SP2 clean, and then the drivers etc. (two times, trying to see what was going on). The funny thing is that I think it was WinUpdate that created the issue; on a clean, non-(Win)updated system without software installed (but I think with nVidia drivers updated) the downloads were fine.
The steps that broke it for me were - Install MSOfiice, run WinUpdate (MSUpdate), install Adobe, hello issues. Just like you said, .cab, .exe., or any kind of compressed folder. I suspect that all files are corrupted in some way shape or form, it is just most noticeable with those types.
Anyway, if somebody gets to the bottom of the issue, please post here.
Oh yeah, and since it works with Marvell it is not a memory issue. Not memory controller either, it's on the processor die. It is either nForce4 chipset (I don't think so, otherwise the problems would have been there all along, and it took updates / software to bring them to the surface), or drivers.
My vote is, I think it is a driver issue, so nVidia would do well to address it. I think it is not readily apparent because it takes some updating to bring the issue up. It seems to be some kind of a conflict or something, but what exactly is conflicting I can not tell without more reinstalls, which are not an option now (I need to work). If somebody has spare time, it would be interesting to install things in order, test, install more, test, update one bullet point at a time, test, etc. Time consuming, but maybe we could get to the bottom of this.