corrupted downloaded files

A recent issue--many (not all) files I download (zip files, program installs
and updates, etc.) wind up being unusable, giving messages like "unexpected
end of file," etc. I have tried both I.E. and Firefox, no difference. The
other computer running XP on my home wireless can download the same files
from the same locations and when I access them with my shared folder, they
are useable on my main computer. (So I know it's not the router or modem).
I've tried a couple of System Restore changes, but can only go back so far
because there's a major Windows update in there.
Strange and quite inconvenient. Any ideas?