How to Keep Your Computer Running Smoothly
Linking to other people's tips isn't exactly what I had in mind for my tips section and I won't normally be doing it, but Lifehack has a nice post by Reg Adkins entitled 26 Tips to Keep Your Computer Up and Functioning which has a very good breakdown of tasks you should do to ensure your PC stays healthy. Having said that, I am going to make a few suggestions of my own and add some comments to those made in the article.
Register your software products
I'd also suggest making sure you register your hardware products too, very useful if you have to return them at any point, though I tend to only do it for high value items.
Run a disk scan to check for hard drive issues
Reg recommends doing this weekly. To be honest, hard disks are fairly resilient and due to the ever-increasing size this turns into a time-consuming exercise. I'd recommend every 3-6 months would be more than enough.
Run a defragmentation program
Again, recommended to be done weekly. My advice would be to move to using NTFS rather than FAT32 (this may not mean much, but when you install Windows it gives you the chance to select one or other. You can also convert FAT disks to NTFS as they are (see instructions here), supposedly without destroying the data (although backup first obviously) with Windows XP. You can read up about the differences and which type is best for you here. To see what type of file system a drive uses, right click it in Windows Explorer (My Computer) and select properties. NTFS, while we were originally it would never need defraging, still does, but again, it's a long process, every 3-6 months should be more than enough unless you're installing and uninstalling apps all the time.
Clear your browsing history and
Delete your cookies
Again, these are suggested as weekly tasks. I don't understand why everyone fears cookies so, yes they can be used for bad, but 95%+ they are used for good and perfectly reasonable functions. You know when you click one of those little 'Remember Me' boxes when you log in, that's a cookie being set. So, if you don't want to find yourself not remembered beyond a week, don't delete them, 3-6 months again. Browser history affects that auto-fill dropdown so when you type in 'goo' it pulls up Google, then you just select it rather than typing in the rest. Why bother deleting it? The addresses time out anyway, if you don't visit an address for sometime the browser 'forgets' it. Clean every 3-6 months if you wish.
Rebuild your desktop
When he says rebuild I assume he means the same thing I do and that means re-installing your operating system. Most people don't do this at all, let alone quarterly, most IT pros are happy with yearly. Again, a long-winded process, so I wouldn't advise too often, most people just don't have the time.
Okay, I didn't mean to put Reg's suggestions down too much, there's a lot of good stuff there, but most people don't need to do all the stuff as often as suggested. Anyway, on to a few of my own.
Only Keep Important Progams in the System Tray
The are where the clock is on your desktop (typically at the bottom right of the screen) is called the system tray. Applications often have icons there to allow you to see their status, launch them or to pre-load themselves to allow faster opening times. Mouseover each of those icons and see what it is. The number of times I've seen people with 10-15 items there is incalculable. You should have around 5 I'd say. You don't need most of the programs unless you are accessing them all the time (or the run in the background). Windows Messenger and Quicktime are two of the worst culprits. Right-click the icon, select preferences, properties or options and find the box that switches them off from appearing there, or in Messenger's case, stop it auto-loading, launch it manually. This will stop them eating up system resources (mainly RAM).
Keep Your Desktop Clear
Another thing programs are prone to do is create links to them on your desktop, some allow you to untick this option when installing, many don't. Delete those icons, and try and keep as little as possible on there, each item takes time to load each time you boot up, so it slows your computer down and once you get past 15 items it no longer becomes a convenient place to find your most used programs because you won't be able to find the button to launch it. By all means use it as a convenient location to temporarily dump files, but get rid of them ASAP. One way to may use of the desktop would be to create a folder somewhere in your system and then create a shortcut to it on the desktop, then things are only a double-click away.
Check Your Startup Folder
On your start menu, under programs, you will find a folder called Startup. Any applications in this folder will run when you switch the computer on. Again, these all add time to loading windows and you being able to use the PC. Many cheeky apps add themselves to this folder so take a look and delete anything you don't need.
Between that lot you should have a fairly stable, solid system that runs well all the time.