Frederick writes:

<<We have a 3rd party
 > software which runs perfectly since 3 years ago.  Right now, the Illegal
 > operation will intermittently pop up,>>

I might have helped a lot if you stated which program is crashing. Others
may have a specific answer. Otherwise, we're just guessing.

More often than not, these conflicts occur because people are running too
many background programs, any one of which might cause this problem.

Run Msconfig and untick everything in the Startup tab. Don't run the
offending prog while anything else is running. See if that fixes it.
Msconfig won't allow you to disable anything vital to the system.

The fact that it's intermittent tends to point to a memory problem. Don't
run Nortons or anything known to take up large chunks of system resources,
and in particular don't run any sort of "System Fix" progs in the
background, which often cause more problems of this sort than they solve,
once the computer starts to go a bit flaky.

Don Penlington
 From the Beach at Surfers Paradise in sunny Queensland.
Computer tutorials, local scenery,  and other things at my website:
http://users.tpg.com.au/deepend/index1.html