The URLs below no more work...
Where can I find update_sys-wme.zip?

Updated UPDATE.SYS for Windows 98 SE, version changed to 4.10.2223
http://www.code.cz/x/update_sys-w98se.zip

Updated UPDATE.SYS for Windows 98 SE, version changed to 4.90.3001
http://www.code.cz/x/update_sys-wme.zip

Best regards
allesok

These are the new URLs for UPDATE.SYS for both 98SE + ME, this time with installers, no need to copy them manually to %windir%\system32\drivers anymore, also posted here:
http://www.msfn.org/board/?showtopic=46581
and here:
http://www.mdgx.com/web.htm

* Unofficial Windows ME Updated CPU Microcode UPDATE.SYS build 4.90.3001 Fix includes most current Intel CPUs:
http://www.msfn.org/board/?showtopic=44388
Direct download [166 KB, English]:
http://www.mdgx.com/files/MEUPDSYS.EXE
How to install UPDATE.SYS Fix on Windows ME:
1. You MUST disable System Restore: open Control Panel -> System -> Performance tab -> File System -> Troubleshooting area -> check the "Disable System Restore" box -> click OK -> reboot.
2. Download + install CoolKill [797 KB, freeware]:
http://www.prowebsitemanagement.com/downloads/coolkill.zip
3. Run COOLKILL.EXE -> right-click on the CoolKill icon in Taskbar Tray -> highlight STMGR.EXE -> left-click on it to terminate this process.
4. Download + install UPDATE.SYS Fix (see above).
5. You may want to reenable System Restore: open Control Panel -> System -> Performance tab -> File System -> Troubleshooting area -> uncheck the "Disable System Restore" box -> click OK -> reboot.

* Unofficial Windows 98 SE Updated CPU Microcode UPDATE.SYS build 4.10.2223 Fix includes most current Intel CPUs:
http://www.msfn.org/board/?showtopic=44388
Direct download [139 KB, English]:
http://www.mdgx.com/files/98UPDSYS.EXE