Not sure if this will help or not ... I also run on Dell optiplex boxes with NT4/SP5. I'm guessing that the "ws" in your description means NT workstation versus server. Mine are servers (even though they don't need to be - they could be workstations). That might be the difference. But in any case, I have not seen the problem you are describing. My point is that perhaps the hardware is not part of the problem.
At this point I am satisfied that it is a system resource issue.
The CPU was very highly loaded (85 - 90%), seems the SetActive command is more sensitive to CPU load than most, and tends to timeout. Segue tech support concur that such behavior is consistent with high CPU load.
When I kill the competing tasks, with CPU load below 10% I have not seen any problem with the SetActive() function.