I think XP needs to be reinstalled at least once a year. :P
No, actually it doesn't.
It just needs a lot of attention!

The box I am on right now is XP pro installed when it first came out. Never re-installed in any fashion, although I did a hard drive change two years ago: I did a disk copy of the original disk rather then a re-install. ( not to mention replaced video card, motherboard, processor, power supply, DVD burner, ..., at one time or another. ) And I can not imagine how many programs I have installed then deleted.

bradlesliect, if this was all of a sudden I would error on the side of caution and BACK-UP everything and then run diagnosis on the hard drive, as previously suggested.
A good idea in any event.

Since you already decided to do a re-install I guess it is moot, but I would consider a few things:

1) there is no one program that will fix all problems
I use anti-virus, regularly run adware and spyware cleaners, reg cleaners, etc.

2) IBID
Not only do you need a versatile toolbox, but many of the tools will yield further results in SAFE mode.

3) Occasionally Defrag
When defrging, make sure the system is stable, then turn off system restore and pagefile
( don't forget to turn them back on afterwards )

4) The more RAM the better
XP used to run fine on 512M ( Vista really needs 2G )
If you are running an anti-virus program, they have grown out of necessity and blot ( added services they think you need ) , so nowadays 1G is minimum.

5) NO program can clean the system as well as a well trained person with the right tools
This may take a little explanation.
This is especially true of reg cleaners, but also applies to spyware cleaners. All too often I have found cleaners saying a program or reg entry is invalid or dangerous when in fact it is needed. You have to know your system if you are going to use these tools. ( example: Norton Utilities, 2002 I think, run from CD, always tries to delete the reg entry for several HP files it can not find. If I allow it to delete these entries the programs that installed these entries cease to work. )
Westin - I have done all apart from Spybot.
Don't forget that Spybot has a tools menu that is extremely helpful, and can supplement what you find in Hyjack-This.
The Windows System Information tool, especially the Software Environment can be extremely helpful in identifying rouge files that are started by some improperly uninstalled programs.

Alright, here is a trick I got from some Microsoft KB article about Ghost Devices.
These build up over time and you can not see them, but they may cause problems, at the very least slow down start-up:
NOTE — this must be done as administrator –

a. Click Start, point to All Programs, point to Accessories, and then click Command Prompt. ( you can use the run-as command here )
b. At a command prompt, type the following command , and then press ENTER:
set devmgr_show_nonpresent_devices=1
c. Type the following command a command prompt, and then press ENTER:
start devmgmt.msc
d. Troubleshoot the devices and drivers in Device Manager.

NOTE: You will have to click Show Hidden Devices on the View menu in Device Managers before you can see devices that are not connected to the computer.
e. When you finish troubleshooting, close Device Manager.
f. Type exit at the command prompt.

6) Don't forget those EVENT Logs!

*** Again, you have to know your system to use these effectively!

For any admin these steps are way too time consuming for a multiple computer system; a re-install or ghost image is much easier and less time consuming ( thus cost effective for the bosses! )

But for a single user, it may indeed help from having to locate all those original install disks and a marathon session to re-install, and may, ( as in my case ) keep your spouse off your back!

Just my thoughts.