Readyboot is a windows feature that optimizes that boot process by using in-RAM cache.
After every boot, the ReadyBoost service (the same service that implements the ReadyBoost feature just described) uses idle CPU time to calculate a boot-time caching plan for the next boot. It analyzes file trace information from the five previous boots and identifies which files were accessed and where they are located on disk. It stores the processed traces in %SystemRoot%\Prefetch\Readyboot as .fx files and saves the caching plan under HKLM\System\CurrentControlSet\Services\Ecache\Parameters in REG_BINARY values named for internal disk volumes they refer to.
It can cause hard-disk thrashing when Windows thinks the PC is idle, that's why some people choose to disable it. However if you're not experiencing any problems I'd avoid that.
Each time you turn on your computer, Windows keeps track of the way your computer starts and which programs you commonly open.
Windows saves this information as a number of small files in the prefetch folder.
The next time you turn on your computer, Windows refers to these files to help speed the start process.
The prefetch folder is a subfolder of the Windows system folder.
The prefetch folder is self-maintaining, and there's no need to delete it or empty its contents. If you empty the folder, Windows and your programs will take longer to open the next time you turn on your computer.
Caches your boot files by creating a readyboot.etl file that gets deleted after each boot, then Superfetch automatically creates a new strategy based on what was loaded previously and what's new, then finally it's used during the next boot, etc, etc.
Even with a lot of ram, its still quicker to have ReadyBOOT enabled because the cached files are read quicker than having to load them each time in memory.
W7 set the file to 20MB and in the event logger, this size is often maxed during boot, increasing the MB's can really help.
I do not recommend a value above 256mb, I have mine set at 128mb.
How to tweak it:
1) Search, Performance Monitor
2) In the Navigation Pane, expand "Data Collector Sets"
3) Click on "Startup Event Traces"
4) Double click "Readyboot" from the list
5) Click on the "Stop Condition" tab and set the size you want (remember not to exceed 256Mb's) click "Apply", then "Ok" and reboot.
You can disable it, but it will re-enable itself upon next restart/boot, unless you disable the Superfetch service and I strongly advise against doing that.