Deleting cookies often makes the computer run better. But why? —Norman Epstein
Windows actively sorts and indexes your Cookies and other temporary internet files. That’s why it creates data (.dat) files in the "Temporary Internet Files" areas; and why those .dat files are normally locked "in use" (and thus hard to delete) when Windows is running; and why they’re regenerated if you do delete them.
In normal use, with default browser settings, you can end up with vast amounts of disk space— hundreds of megs— of temporary files, much of it a churning mass of snippets, images, web pages, cookies, and such, all requiring the attention of the operating system. It’s not a trivial task for Windows to manage all that, and it can exact a noticeable toll on performance.
On the other hand, when you clean out the temporary files, Windows has that much less housekeeping to do, and less data to actively manage. Indeed, your PC may run perceptibly faster.
That’s why I recommend a two-pronged approach: First, reduce the amount of, um, crud that gets into the Temporary Files area in the first place. In IE, click Tools/Internet Options, and in the "Temporary Internet Files" area, click Settings. If you have a high-speed connection, set the "Amount of disk space to use" to 10MB. If you have a dial-up connection, try a 25MB setting. That’s usually all you’ll need; although you can always come back and increase the temporary storage allotment if you need to.