There's *tons* of information on the Web about all aspects of Windows XP -- everything from tutorials to tweaks to troubleshooting. Windows built-in "Help and Support" is also a good place to look [Start > Help and Support > Search]. And don't forget Google.
"Patching" Windows has become an essential part of managing online security. Step 7 of my security plan, and the page on "Patching and Upgrades" have the information you need.
It's best to have a fresh backup before updating Windows. Sometimes updates or upgrades just don't work out. You could be left with a computer that won't start. How will get at your critical files then?
Windows often has shutdown/restart/logoff problems -- especially on laptops. Microsoft's "User Profile Hive Cleanup Service" -- actually a little program -- can often fix shutdown problems automatically. Windows Shutdown Center (WSC) is a good source of solutions to shutdown problems.
Not all conventional wisdom about tweaking Windows performance is correct.
You may have never had trouble with Windows Product Activation (WPA), but Microsoft has now complicated your world with Windows Genuine Advantage (WGA). [background]
Those pesky icons in the system tray reveal many of the culprits. Each one represents a program that is running. They're in competition with each other, and with legitimate programs your using. Your computer will run better if you clean out some of the clutter (guide applies to XP too).
There are also a bunch of useless "services" that Windows XP starts. You'll almost certainly never need some them. The proper tool for managing Windows XP services is located at Control Panel > Performance and Maintenance > Administrative Tools > Services. You can also get there this way: Start > Run > enter services.msc > click OK.
I've created a simple service settings table showing what I recommend for most users, along with the settings that I use to fit the way I use my Windows XP computer. Black Viper's website (a superb site, don't be put of by the Gothic appearance) has a comprehensive guide for reducing the number of seldom or never used services.
If you're going to make changes, do so a few at a time, and keep a record of what you've done. If something quits working after you make changes, you'll never know which one it was if you make them all at once. You can always use the default settings to get back to the original configuration though.
Remote Procedure Call will make your system unbootable if you disable it. Do not disable RPC by using the Services method, the hardware profiles method or any other method.