Russ:
My initial thought is to do all the proper housekeeping first rather than jump in with a reinstall. There's a lot that can lead to a slow-system, and much can come from inefficient management in various areas ranging from simply having too much baggage installed - unused/unneeded programmes, excessive fonts and more - to irregular maintenance such as clearing-out unnecessary files, defragging regularly, that kind of thing. Some things may not be much in isolation, but the cumulative effect can bog things down.
Next would be a check for ads/spyware etc, but I see you have done that. It is always useful to run two different spyware programmes however, as one can catch what another might miss; maybe you already do, but if not Ad-Aware SE Personal and Spybot make a good start.
To reinstall XP:
I am not an expert but I have done this successfully a couple of times, the last time just yesterday ....
Briefly, you can install XP either afresh or over itself - the first wipes everything and returns a virgin system, the second retains your setttings, programmes, docs etc ..
Go here for a walkthrough ..
www.michaelstevenstech.com/XPrepairinstall.htm
Please note the warnings.
Also note that the word "Repair" crops up in both the 'virgin' and 'over' options and could be misleading at a glance. Be sure to pay close regard to step-3 of the walkthrough stressing NOT to choose the repair option using Recovery Console.
And be sure you have your Product Key as you need to enter that part way through the process.
Reinstall Service Packs afterward, and get updated via MS website after activating the firewall, and you should be back to clean-and-fresh.
Ohh, and you might find a need to reinstall things like DirectX etc. afterward, but those will be prompted-for according to need as they arise - no big issue there.
HOWEVER
If you have any external drives on your system you might want to reconsider, or at least wait till a solution emerges to the problem that I have just started a thread about ... look for "External hard drives not *seen* after XP Home reinstall"
But if you don't have external HDs, what you want to do is very straightforward and effectively gives your system a spring-clean ...
I expect there will be more responses to your question, and you should wait a while and then take note of other useful remarks.
Good luck if you go ahead, though in reality it is very straightforward and shouldn't be any problem aside from the caveat about external drives if that applies to you.
Shel