This is what I got from my bud at McAfee about the 4.0.5 Engine, never heard of problems with the 4.5.x engine.
I am cureently running 4.5.1 with the .4153 DAT file and have no trouble, we have it on Win95/98 and 2K, no troubles so far.
See if you can upgrade to 4.5.1 if that makes any diff.
Here's the warning we got.
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The 4153 DAT files released on Wednesday, August 8th are now having issues
with the 4050 engine. These issues can/will lead to issues ranging from
IPF's to Services crashing; and, in some cases, we have seen some machines
completely crash. In order to ensure this does not happen to you, please
verify that your McAfee Anti-Virus products are running an Engine newer than
the 4050 engine. If you are currently experiencing these issues, running
the 4153 SuperDat will resolve them as it will upgrade both the engine and
DAT files.
Information concerning the EOL of the 4050 engine and why these issues might
occur may be found by following this link and scrolling to the bottom of the
page:
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Chadda