Saggy your symptoms are exactley the same as I get now and then. You must have a stinking bloody idiotic Lithion Smart Battery the same as I have.
What it happened I dragged the cursor down to the battery icon in the taskbar tray (took all bloody day to get there!) where it showed 0% charging. This of course is bullsh!t as a battery cant go from 100% to 0% in 1 second. It might be because of the stupid so-called Smart Battery and its Windows interface and the battery reports b*llsh!t to W98 resulting for some reason in the computer slowing down. Refreshing the battery didnt help either.
Cure? The only I know of through experience is just turn the computer off. Remove the battery and put it back again ensureing its fully secure. Leave the computer off for a while. Reboot. Check battery status. If its 100% charging you should be ok but dont physicaly push and shove the computer itself around. Keep an eye on your computer battery light (if you have one). If it flickers check the battery icon again.
Id be interested if someone else has a better cure.