If you're not using this much-hyped Vista feature--which supposedly speeds up Vista by caching memory to a flash drive--it's actually slowing your system down a tiny bit. (And if you are using ReadyBoost, it's probably still a drag on your PC. For an explanation, read "ReadyBoost Flash Drives Lack Significant Boost.")
You turn off ReadyBoost in Services. If you aren't already there, click Start, type services, and press Enter. Find and double-click ReadyBoost. In the 'Startup type' drop-down menu, select Disabled, and then click OK.
This one is a real trade-off. Turning off Vista's indexing will slow searches to a crawl--I'm talking minutes, not seconds. But ditching this convenient feature could very likely speed up your general PC use significantly.
In other words, turning off indexing will help your PC's performance only if you seldom search by file content, or if you use a third-party search tool such as Copernic Desktop or Google Desktop (in which case you probably have two indexing routines running at the same time, which is an even bigger waste).
If you match either of those descriptions, turn off indexing by clicking Start, typing services, and pressing Enter. Find and double-click Windows Search. In the 'Startup type' drop-down menu, select Disabled, and then click OK.
If you work on files stored on a server somewhere, and you can't depend on that server always being available, Vista Business and Ultimate's Offline Files feature makes your life easier by copying the files to your hard drive and keeping them synced.
Of course, that sort of thing isn't for everybody, which is probably why Microsoft didn't include Offline Files in the Home editions of Vista. But if you have Business or Ultimate and still don't need Offline Files, turn it off by clicking Start, typing services, and pressing Enter. Find and double-click Offline Files. In the 'Startup type' drop-down menu, select Disabled, and then click OK.