Right click "My Computer"
Select the "Hardware" tab
Click the "Device Manager" button in the middle box
Click the "+" next to "Disk Drives"
Right Click the hard drive and select "Properties" from the drop-down
Select the "Disk Properties" tab
Tick the box if it isn't ticked to enable write cacheing
Well that's how to do it, but to be honest, I'd be surprised if it wasn't already switched on. Have you tried all the usual stuff like a de-frag, disk tidy up etc ?