Okay, first things first...you'll want to run Disk Cleanup to get rid of any unnecessary temp files on your hard drive. I don't remember if WinME even has Disk Cleanup so if it doesn't, just find the Temp folders and delete everything in them.
Then you'll want to do a full system scan with your anti-virus with updated definitions. Follow with scans for spyware, trojans and hijackers.
Then run ScanDisk and then Defrag.
Do you tend to install and uninstall lots of software? If you do, the Registry is probably clogged up with unused keys. Might want to run a Registry cleaner too.
Do you have a lot of programs running in background? Do you have lots of icons next to the clock? If you do, then you have a lot of background programs...you might want to disable the unnecessary ones to free up some system resources. You can use msconfig to stop some background programs from loading up with Windows.
And adding another 128MB of RAM might help a bit, although I think 128MB was more than enough for WinME.
Cricket