The following suggested methods of cleaning up your PC and improving performance applies to PCs running Windows XP, though several programs may be available for Vista. Every program and method listed below is free. Be sure to differentiate between the freeware and trial versions of the software when you visit the below links (you want the freeware).
Side note: If you are considering a new PC, you may want to delay if possible until the release of Windows 7, which is the next iteration of the Windows operating system software. It will replace Vista and, supposedly, offer a number of performance and compatibility improvements.
Step 1 - If you have never run the following program, do so once per PC, particularly new PCs including those with Vista. It is completely free and very simple:
http://www.pcdecrapifier.com/
Step 2 - Install and run Spybot Search & Destroy once every month. The program will also ask you if you want to install, "Teatimer," a utility that monitors changes to your system registry. Depending on the user, it can be either annoying or informative. You may want to forgo its installation to save on system resources and pop-ups:
http://www.safer-networking.org/en/download/
Step 3 - Install and run RegCleaner whenever you run Spybot. It cleans your system registry of old and unused entries for programs that have been uninstalled or have become outdated. Running this program should boost your boot-up time:
http://www.tweaknow.com/download/RegCleaner.exe
Step 4 - Defragment your hard disk(s). This will improve disk reading times, and should therefore decrease the time it takes to load programs:
a) Go to your Start Menu -> Accessories -> System Tools -> Disk Defragmenter
b) Click to highlight the disk you wish to defragment (if there are more than one, defragment each one, one after another)
c) Select, "Actions" from the Window drop-down menu, then Defragment
d) Allow the program to run, it can take quite some time (sometimes as long as an hour or more, depending on the size and speed of your drives, how fragmented they are, and other factors)
Step 5 - Turn off visual effects you do not need (XP only). Removing effects like menu fading will improve overall Windows performance:
a) Go to your Start Menu -> Settings -> Control Panel -> System
b) Click the, "Advanced" tab
c) Under, "Performance," click the "Settings" button
d) Click, "Adjust for best performance," Then hit okay.
Step 6 - Increase your page file size (XP only). When your memory runs out, programs write to files called, "page files" on your hard drive. While you want to limit the use of page files in general by installing as much memory as possible in your machine, when you do use page files you do not want to run out of room:
a) Go to your Start Menu -> Settings -> Control Panel -> System
b) Click the, "Advanced" tab
c) Under, "Performance," click the "Settings" button
d) Click the, "Advanced" tab
e) Make sure that both boxes for, "Programs" are checked off.
f) Under Virtual Memory, click, "Change"
g) Double the Initial and Maximum size; do not round: Literally multiply the current total by two. If the, "System Managed Size" box is checked, leave it. If "No Paging File" is checked, check, "System Managed Size" instead.
Additional Tips
Turning off your background and screen saver will also save some system resources and speed up your PC a bit.
Installing the latest drivers for your graphics card (if you have a card available ) may also improve everything from operating system to game performance. Check your system manual for the type of graphics card you have installed, if any, and visit the manufacturer's home page for an updated driver.
If you need an Antivirus software, try AVG Antivirus, which is completely free, not to mention uses less system resources than competing antivirus software:
http://free.avg.com/download?prd=afe
Enjoy a faster computer.