Is your WinXP box running slow? I'll give you some information on speeding it up, without upgrading your hardware.

First things first. Disable any unneeded services:
Go to Start>>Run, and type in 'services.msc'.
This will open the Windows Services Manager.

Now, some services may be stopped, and set to manual...this doesn't give you too much reason to disable it, unless you notice a difference.

There are many service you can disable, the most prominent being the Indexing Service. This service indexes your harddrive, constantly, keeping a database of all the files on your harddrive. Now unless you are always searching for things, then this is a performance hog.

Alerter - enables Administrative alerts.....garbage
Automatic Updates - are you that lazy?
Error Reporting Service - asks you if you want to send a report to MS...garbage
Fast User Switching Compatibility - unless you are always switching users, you don't need this...you could actually log off.
Fax - does anyone use this?
Help and Support - use this if you need Windows help...otherwise, just use Google.
Human Interface Device Access - support for keyboard hot buttons, and other such crap
IMAPI CD Burning COM Service - why use XP's embedded burning when you can use Nero?
Indexing Service - explained above, garbage. Search will run a bit slower...who cares. Use Google Desktop Search for texts, only runs when PC is idle
Machine Debug Manager - supports the debuggers in MS Visual Studio apps, disable if you don't use Visual Studio.
Messenger - this is not Windows Messenger, this is for "net send" on the command line...trash.
Net Logon - If you aren't on a domain, trash this.
NetMeeting Remote Desktop Sharing - Has anyone *ever* used NetMeeting? This may affect Remote Desktop as well, I'm not positive.
Network Connections - This will make managing the network/dialup difficult for you, but it is a good security measure to disable it on a large network.
Performance Logs And Alerts - More event logging that is unneeded by 99% of the planet.
Portable Media Serial Number Service - do you use files with Digital Rights Management? If not, get rid of this. But if you ever have problems burning music, or copying it elsewhere, enable it.
Remote Desktop Help Sessing Manager - Unless you use Remote Assistance, you don't need this.
Remote Registry - why would you want people on another PC editing your registry
Removable Storage - if you don't use USB disks and the like, then get rid of this.
Secondary Logon - This will not allow you to run executables as another user (Run as...), but you should be an admin already, right?
Shell Hardware Detection - Used for AutoPlay options...unneeded, quit being so damned lazy!
Smart Card - if you don't have one, you don't need this
System Event Notification - why so much damn logging on a Windows box!
System Restore Service - I guess this depends on how much you screw up the PC. disable this last in case you stop a service you shouldn't have, you can roll back.
Task Scheduler - Do you use the Task Scheduler...no?!
Telnet - allows others to logon remotely and run code...why?!
Themes - This will get rid of many of Window's bells and whistles that make it look pretty....your RAM is at risk!
Uninterruptable Power Supply - if you don't have a UPS, then toss this.
Windows Firewall/ICS - If you have a better one (ZoneAlarm, Tiny, Portzilla, etc), then disable this.
Windows Image Acquisition - to get images from digital cameras and scanners.
Windows Time - syncs your system time with a server...but you should always change time via BIOS anyways.
Wireless Zero Configuration - for Wi-Fi access, unneeded if you don't have a wireless network

Go here for a description for every Windows service - http://www.ss64.com/ntsyntax/services.htmla
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*whew*, that is a long list. alright, that takes care of just about everything.
a few more, changing the views of many things will help. Less complicated views will allow more RAM for programs. Folder views don't have to be in a web view, nor does Active Desktop need to be enabled.

Go to Start>>Run, and type 'msconfig'. Click the startup tab, and disable things you don't need. If you don't know what it is, I'd recommend you find out, or leave it alone.

**** - Spyware, adware, and viruses can be dentrimental to your system's performance, scan weekly.
Use software such as Ad-Aware regularly. Spybot S&D is a definite complement to Ad-Aware, and allows the immunization of the system.
http://www.pandasoftware.com/activescan/ - This is a free online virus scanner, this is arguably the best one out there.

**** - Defragment your drives, once a month....more often if you are a download-freak.

**** - Don't use a desktop background....just keep a color, any will do.

**** - Alternatives to some MS applications can also boost your performance. You can use a low profile browser, such as Mozilla Firefox, if you really want to tweak all performance out of Windows. Using Wordpad in place of Word can make things run quicker when it comes to word processing. This goes the same for smaller media players in place of Windows Media Player.


Now, this is a relatively comprehensive list. If you have any additions, feel free to post them, PM me to add them, or even just bitch and complain about it...who knows.

Now, when it comes right down to it, you don't even need sound enabled (another service)...that's disabled by default on Windows Server 2k3.


Cheers

A_T

Additions:
author="Pawelsoft"
Where is TuneUp Utilities 2004 mentioned in your advice?
author="AxessTerminated"
Also, any type of tweaking utility could help, providing you know what you are doing.
Xteq Xsetup
TweakUI
Tune Up Utilities