Recently the term Vampires has been applied to electronic devices that consume energy even when they are powered "off" such as TV's, computers, and battery chargers. Numerous articles have appeared that urge users to unplug devices when they are turned off, or to use a power strip for your computer & accessories so you can power-down & then just flip the switch. These can be effective in reducing energy usage in many cases.
In a similar vein (no pun intended) there is another type of Vampire that can be lurking within your computer, in the form of background services that run continuously and steal cycles from your CPU and thus diminish your computing experience.
One example is the software that comes with a digital camera - usually there will be a background service that continually monitors when the camera gets plugged-in & then launches the photo-transfer software. But think about that - you know when you plug in the camera, so why not just launch the software after you plug in the camera?
Other gadgets like smart phones, GPS, MP3 players etc. have similar background services that you don't actually need running unless the gadget is connected.
Furthermore, many software programs today have an automatic update feature - a small program that runs in the background to periodically check for software updates. These auto-updaters will slow the computer at boot time & in many cases continue to lurk in the shadows. Windows Update is a well-known example of this, but products from Adobe, Symantec and many others will also have this behavior; some of them will pop-up an intrusive message that an update is available & you should install it. The good news is that products from major vendors such as these will have an option to turn off the auto-updating (but places the burden on the user to periodically check for updates; this is still a good idea, especially with anti-virus software).
The bad news is that some software doesn't have that turn-off option. In fact, some will continue to check for meaningless updates even after you have uninstalled the product. If you have technical skills, you can use msconfig (built-in to Windows) or free utilities such as Auto Runs and Process Explorer (search for Microsoft Sysinternals) that can help identify & turn off these vampires. Other useful utilities are HijackThis and Belarc Advisor, but they require a bit more technical knowledge to interpret the results.
Last but not least, use caution when installing "free" software downloads; a few of them install additional programs that "come along for the ride" like custom toolbars & add-ins, and you don't always have the option to de-select them. Before you install anything at all, do a Google search to find out what others have said about the product.