Fierce competition between Intel and AMD has pushed prices for many CPUs well below $100, making a processor upgrade more practical for older PCs.
Start by finding out which CPUs your PC's motherboard supports; check the device's documentation, or visit the manufacturer's Web site.For Intel CPUs, identify your chip set with the Intel Chipset Identification Utility; for either Intel or AMD hardware, use SiSoft's Sandra 2007 Lite.
Three scenarios make a CPU upgrade worthwhile: Moving from a Celeron to a Pentium, switching to a version of your current CPU that's at least 50 percent faster than the one you already have, and shifting from a traditional processor architecture to dual-core.
In Video: How to Replace Your CPU
If you can't upgrade your processor, think about buying a whole new motherboard. As we went to press, a dual-core AMD Athlon 64 X2 3800+ CPU costs just $79 online, and an Asus A8V-XE motherboard is priced at $50. Look online for motherboard-CPU combo offers, which are often an even better deal. Note that you can reuse your system's old memory if your new motherboard uses the same type and speed (DDR, DDR2, or whatever).
CPU and cooler: Check Sharky Extreme's weekly price list to get a feel for CPU prices. For background on CPU coolers (heatsink/fan combinations), visit online forums; the fan that came with my Pentium D 820 CPU sounded like a 747 preparing for takeoff until I replaced it with a $50 fan from Zalman.
Thermal grease: Your CPU may come with a small tube of thermal grease, also known as thermal transfer compound; more likely the vendor pre-applied some to the bottom of your new cooler. If no thermal grease came with the CPU, however, buy a tube from your local computer store; this compound is essential for keeping today's hot CPU's running cool.
Tools: You'll need a nonmagnetic, Phillips-head screwdriver and perhaps some other tool to open your case (some older CPU coolers are held in place with flathead screws). Another worthwhile piece of equipment is a $10 grounding wrist strap to protect your PC's expensive circuitry from chip-frying electrostatic discharges.
Caution and patience: PC components are extremely sensitive. Work slowly and deliberately, and never forcefully push, pull, or twist anything inside a PC case.
Open the case: Unplug the case and place it in a position that offers you comfortable access and good lighting. Ground yourself by touching a metal object (other than your PC's case) or by donning your grounding wrist strap, and then remove the case cover.
Take out the motherboard (optional): Installing cooling fans that come with support posts for attaching to the motherboard often takes some mechanical finesse. If you remove the motherboard from the case, you'll find that your CPU and cooler are easier to attach.
Detach your old CPU, heat sink, and cooling fan: Read all the installation and removal instructions that came with your CPU. You can remove most coolers either by flipping a lever (socket 939, 940) or by giving four securing posts a half-turn (Intel socket LGA775). To remove a CPU, lift the small lever on the side of the CPU socket to loosen the clamping mechanism, and then gently pull the CPU out.
Insert the new CPU and fan: Don't take the new CPU out of its protective envelope until you're ready to install it; and and when you do remove it, handle it only by the edges. Align the chip with the socket on the motherboard, insert it, and flip the lever to clamp it into place. Fasten the new fan and heat sink. The first time you restart the PC, you may have to go into the PC Setup program in your system's BIOS (look for the on-screen option before Windows loads), and select a new clock frequency. (Again, read the CPU's instructions before undertaking this operation.)