You say nothing is overclocked, but do you have the "coolbits" option enabled giving you the ability to set clock speed on the nvidia card? Here's a blurb from the Geforce Faq:
Q. When I resume from monitor power saving modes in Windows 9x/ME, I get display corruption and / or instability problems, or my card seems slower than usual. How can I fix it?
Thanks to Jarhead for pointing out this problem.
In the 6.47+ drivers under Windows 9x/ME only, there is an issue for some (not all) systems when using monitor power saving modes. On these systems, if you allow the monitor to go into standby mode, the drivers may try to reapply any overclocking settings ON TOP of your current settings when the monitor resumes from the power savings mode. This causes display corruption and instability, and can seriously damage your card if the card is used in this overclocked state for a long period of time.
For the most part, this only affects people that are actually overclocking with Coolbits, but it has been reported to happen even if Coolbits is not enabled.
To see if your system is affected by this problem, perform the following steps:
Right click on your desktop, click Properties / Screensaver / Settings / Power Schemes, and set your monitor to shut off within 1 minute. Sit back and allow Windows to shut off your monitor. Wait about a minute and then move the mouse a little to wake the system up.
If you can see display corruption or the computer crashes, you have got the problem. Do not enable the monitor power savings modes or go into Standby mode on this system.
If you are using Coolbits to overclock, and the system does not display any problems after you perform this test, check the NVIDIA hardware options tab to see if the overclock setting you have previously made is still there. If it is, then the bug does not affect you.
If you are not using Coolbits to overclock, and the system does not display any problems after you perform this test, wait for the power saving mode to kick in and then move the mouse 5 or so times more and see if you then get the problem. If you do not then it is almost certain that the bug does not affect you.
This problem is eliminated for some after version 6.50, but not for others. You should test your system before enabling monitor power saving modes.
Alternatively, you may find that when you come out of power saving modes your 3D card seems slower because the clock speed has not been restored correctly. If this is the case, you can use NVresume to reset your clock speed when you come out of power saving modes - just put it in your Startup group. You can download NVresume from the following website:
Get NVresume here (Nvresume is a freebie...give it a shot)