Old myths never die
Can we get a Snopes.com for technical urban legends?
I should totally do that on my wiki!
People also think that cleaning the Prefetch folder helps, or that not cleaning it will somehow lower your performance. I would have thought a site like "windowsnetworking.com" would have been a bit more professional about these things, but I can't have everything. They said, and I quote (emphasis theirs):
Prefetch when unattended can also slow down your system. This is because over time
* XP will retain a copy of a portion of a program in the prefetch folder even if you only use it one time, which is not good. Since you may not use the program again, you may impact the performance of your system by having portions of a program you do not use loaded in your system's memory.
* XP systems with very low hardware resources (such as memory and hard disk space) will definitely be affected by an over-bloated prefetch folder.
This is bull. Let's break it down:
* XP will retain an index of the locations of files needed by a process in the prefetch folder even if you only use it one time, which is not a big deal. Windows XP automatically prunes old entries after a 129th entry (or more) needs to be created. At worst, you're going to lose less than 20 MB of hard disk space (my prefetch is currently using 4MB).
* XP systems will very low hardware resources (such as memory and hard disk space) are already at a disadvantage, but without prefetching, it would be even worse. There is no advantage in deleting prefetch entries…. and the only time prefetch entries need to change are when the files they are prefetching have changed (such as being upgraded by a Windows Service Pack or hotfix). And as previously stated, they will automatically be pruned to just 128 entries. At least someone out there knows what they're talking about. If you delete all of the prefetch files, they have to be prefetched *again* (which comes at a performance loss), and doesn't that seem like a waste of time?
* Windows doesn't load things in the Prefetch folder unless you're actually attempting to run the program, so claiming that Windows erroneously loads files in there that no longer exist (and thus need to be cleaned) is also patently false.
* Ed Bott said: It doesn’t actually preload anything into RAM until you run the program, and when you do that, prefetching only works if you have RAM to spare.
Notes from Microsoft:
* Once every three days, by default, Windows XP will perform a partial defragmentation and adjust the layout of the disk based upon current use. The files to be moved are written in the file Layout.ini (found in the Prefetch directory under the System Root directory). 
(Dial-a-fix's "Process Idle Tasks" tool forces this to happen sooner than 3 days).
* Windows XP also uses prefetching when launching applications. The files and the contents of the files accessed by each new process are observed and recorded. No prefetching can be done for the first launch of an application, so first launches are often considerably slower than subsequent launches. About 85% to 90% of the improvement is realized after just one launch of an application, with the remaining speed improvement coming after the system has had an opportunity to adjust the disk layout with information specific to this application. 
Also, search for "prefetch" here.
Anyway, these technical myths are extremely annoying to technicians (such as myself), who constantly have to disprove these dubious claims to customers.