This is a really useful feature of Vista, that I though I'd share with the forums even though its pretty well known... sometimes things go un-noticed
Insert your flash drive into a USB port. If you have auto play enabled, there should be an option in the menu that appears that says, "Speed Up My System" that will allow you to enable ReadyBoost and skip to the final step (step 6).
Open the My Computer tab.
Find the drive letter that your flash drive is occupying. Your computer will automatically assign the next available letter in the list when you plug it in. If you have one hard drive, and two disc drives, this means that your next drive letter is F. However, if you have multiple drives (hard drives, disk drives, removable disk drives, etc), or partitions, the letter will be further down the list.
Right click the drive letter, and choose properties.
Look for the ReadyBoost Tab, and select it.
Look for the option to enable the flash drive as RAM, and allocate the amount of space on the drive you wish to use as RAM. Remember, most systems (even newer ones) max out at 4GB RAM. So if you have a drive larger than that, or 2GB RAM already, you need to remember this limitation so as to prevent damage to your system.
This guide was copy+pasted from
How to Use a Flash Drive as RAM | eHow.com
I take no credit for this...