Well if you just wanna do it temporarily go to "C:\WINDOWS\Prefetch" and delete the files in there. Although they come back just by opening a program which destroys the point of deleting them really. I'm guessing you'll have like 200 files in there
(I get around 70 within a few hours of it being turned on, by opening MSN, Firefox, FL, Notepad, Winamp etc. etc.).
Anyway, go to Run (Windows Key + R), type in "regedit" then navigate to: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters
Under this key you should see a value named: EnablePrefetcher
It has 4 possible values;
0 - Disabled : The prefetch system is turned off.
1 - Application : The prefetch only caches applications.
2 - Boot : The prefetch only caches boot system files.
3 - All : The prefetch caches boot, and application files.
Pick option 2. 0 is just stupid and 3 is the default (if I remember). Restart, the second time you restart it'll be faster.
So to be honest I didn't turn it off. Whoops.