Systems don't physically "slow down" with age. But they do need regular maintenance, just like a car. The main culprits if your Windows machine is getting slower and slower are registry cruft, fragmentation, a glut of tempfiles, and malware.
Best registry cleaner:
EasyCleaner by ToniArts, freeware
I've hand-vetted what it wants to clean out, and in 12+ years have never seen it make a mistake. Run this religiously once a week.
Defragmenter: the safest route is to use the Windows defragger. However, depending on a bunch of factors, defragging can destroy data, so make sure you have good backups first. Every PC should be defragged once a week (or at the very least, once a month), whether it claims to need it or not. Fragmentation is the #1 cause of slow, unstable systems, especially with regard to web browsers.
You can considerably reduce future fragmentation by putting your browser cache on its own partition, or on a ramdisk.
Dataram RAMdisk is free for personal use; I make it 200mb and point my browser cache at it. (The Mozilla/Firefox/SM family are really
dreadful for making a mess with their cache, so it's best isolated.)
Tempfiles lurk in places like
C:\Documents and Settings\user name\Local Settings\Temp
Regularly inspect this folder and kill off zero-byte files, and whatever else looks like junk. (Usually it's fairly obvious, by the garbage name.) There should be few or no files left when it's "clean".
If you use IE,
clear its tempfiles regularly, and with older versions be sure to check "delete offline content" or it really won't do much housecleaning.
Malware can also put the slog on your machine, because it's busy doing Something Else for someone else's benefit.
As to malware protection, McAfee and Norton are not good choices for home users, and all by themselves can slow down performance by several orders of magnitude (no, I'm not exaggerating, I've seen both make a 3GHz machine take 20 seconds to respond to a mouse click).
FProt and
Avast are not nearly as piggy, and both do a better job. Also recommended is
Microsoft Security Essentials (which was a high-end enterprise product that MS bought).
It's a good idea to check
Task Manager to see what's going on. Get familiar with the list of "Processes" after a clean restart and when you run your everyday programs, and watch what each uses. (Note: "System Idle Process" is a measure of what's NOT being used.)
Also, beware of add-ons like toolbars, "Incredimail", and other "freebies" -- many are system hogs and spyware to boot.
Another thing to watch out for are "Control Centers" such as are installed by a lot of printers and video cards -- these are often real hogs, and seldom necessary.