- Clean up the disk. Uninstall unneeded programs (especially those that run at startup and/or put something in the system tray), run Disk Cleanup, and defragment the drive. This is a good first step that will almost always take a few seconds off boot time and application loads for any computer.
- Run a full anti-virus and anti-spyware scan. You never know what's lurking on your machine, and these anti-malware tools can help you get rid of other PC-clogging detritus on your PC, even if it's not harmful.
- Clean up the registry. This is controversial, as some experts claim registry cleaners don't really help. I've seen evidence to the contrary, so I recommend doing it if you've got a major slowdown. CCleaner is free and worth running.
- Delete old network connections. Your computer may be trying to connect to shared hard drives that no longer exist. In Windows Explorer right-click on any network shares you don't actively use and click Delete. Under Tools, also click "Disconnect Network Drive" to see if there are any others lurking about.
- Stomp auto-starting programs. Click Start > Run and type "msconfig" at the prompt. Click the Startup tab and look at all that junk that loads when you launch your PC. Do you really need "Adobe Reader Speed Launch"? Probably not. Turn off anything else that looks useless, but be careful not to disable Windows system components.
Those are the easy and free things you can do. If your computer is still slow you need to move on to the bigger guns.
- Upgrade RAM. This is the one killer trick that will make almost any computer run faster. With an older PC, you will rarely have enough RAM to run today's memory-hogging operating systems and applications, and adding a high-capacity stick or two of quality RAM will give you a quick speed boost. Adding RAM is fairly simple, even for a novice, and you should be able to do the job in 5 or 10 minutes.
- Reinstall Windows. If the above tricks haven't helped, it may be time to wipe the slate clean and start again, reformatting your hard drive, reinstalling your applications, and restoring your data files from a backup. You'd be surprised how much more responsive a freshly reinstalled Windows system can be, as you've wiped out years of temp files, garbled registry entries, old versions of software programs that have been upgraded repeatedly, and all sorts of other electronic junk. Reinstalling is easy if you have the "recovery disk" that came with your PC, and only a bit more involved if you're using a retail copy of Windows XP. Just be sure you back up everything you want to take with you before you pull the trigger!
- Upgrade your hard drive. This is a more complicated solution, but if you're reinstalling Windows (per the prior tip) you might consider upgrading to a bigger and possibly faster hard drive, too. Hard disk storage is a performance bottleneck on every machine, and magnetic disks degrade over time. Some performance issues could be caused by a failing hard drive, even, and upgrading to a new model could really put some zip back in your system. As a bonus, you can use the original hard drive for backups or occasional storage, if you put it in an enclosure.