Here is a snip from the guide:
"Enable ReadyBoost on a Non-Vista certified USB drive.
1. First things first. Plug the device in. Ignore AutoPlay if you have the enabled, and go to Computer in the start menu. When it pops up, right-click the USB stick and select properties.
2. Click on the Readyboost tab on the properties menu and check Do not restest this device. Click okay and unplug the stick from your computer.
3. Open the registry and go to: HKLM (Local Machine) -> SOFTWARE -> Microsoft -> Windows NT -> CurrentVersion -> EMDgmt. You'll have a list of USB devices the computer has encountered, one of which should be your USB stick. Click on it.
Here there's a few details you need to edit. Double click on Device Status and change the value to 2, then ok. Do the same for ReadSpeedKBs and WriteSpeedKBs, changing their values to both 1000
4.Now all that's left to do is put the stick back in and once again go to the device properties (Computer > Right-click on drive). If you look under the Readyboost Tab, you'll be able to now select Use this device."
You can check it out over here.