HELP ! Virus, Trojan or what ????

Hi there,

Has anyone come across the following problem ?
I am using Windows ME (of course).
The reason I ask is I want to know what is causing it .. and a
permanent cure !

I download all my programs (Windows exe install programs for Palm and
PPC amd also Windows CE) to the seperate D drive on my computer .. it
holds nothing else but programs.

A while ago when I tried to install a program, I got a message saying
that the file was corrupt and that I should download it again and
after downloading the file again everything was OK !
Even the file sizes were the same so I was puzzled.
Even worse when I started to check some of the other exe's the same
thing happened ... some 2 weeks later I had managed to go through all
the programs on my D drive checking and re-downloading when possible,
deleting all the corrupt exe's at the same time, an estimate is that
over 80% of the exe's were corrupt !

At this point I replaced the C drive as it started to loose surface
and was causing problems, so Windows ME and all the programs were
replaced from the original installation disks .. another long job !
I would like to point out that I have ALWAYS had the latest version of
Norton AV and also run Zone Alarm all the time, I also run Ad-aware
and Trojan Remover etc.
After re-installing I ran all these again and they found nothing.
As an extra precaution (or so I thought) I made every exe on D drive
"Read Only" with the Properties, so I thought it would be OK and
nothing would "attach itself" to the exe.

All has been OK for about 5 weeks, the after a scan disk start up (one
of many - as usual with ME), I know find I have loads of corrupted
files again .... I really can't believe this !!!!!

Help please !!!!! .. or some sort of clues .. or any suggestions how
to recover the files without completely downloading them again ?

Thanks in advance,

Steve