My sister has one of the white Macbook's, and it is running so slow lately. I repaired disk permissions, repaired, and checked the activity monitor to see if anything was running that shouldn't be. It's only got 1 gig ram, but it used to run fine with that amount. What else could be the problem?
I'm pretty sure it's an early 2008 model.
http://support.apple.com/kb/SP5
1. backup Home folder to external hard disc.
2. reinstall MacOS (just the OS not uncheck all the extras in the customise button) select archive and install - not saving network settings.
3. restart and after reentering your details (not necessary if you have a me.com account) go immediately to SW Update and allow updates to be installed.
4. you have a fast MacBook once more.
5. you may need to import any additional fonts you've installed from Previous System folder
All this is a short and satisfying operation.
When you are totally happy with the way everything's running you can delete 'Previous System' folder - I'd give it a few days before doing this though.
hope this helps a little.
it may seem an extreme remedy but it's the most complete one I know (apart from backing up, erasing HDD totally and rebuilding from scratch)
ps - you could install Snow Leopard if MB is still running 10.5
ps #2 - you could also entertain the purchase of 2x1gb memory sticks but if it was running fine before then it'll be fine again after the above remedy