The usual is that a worm has invaded the system. Without SP4, continual parasite removals and such, the worms can invade such systems even if you install an antivirus. I'd use the Anti-Parasite Suite and see what you find.
Parasite removal programs.
http://reviews.cnet.com/5208-6132-0.html?forumID=32&threadID=1313&start=0 notes the five scanning tools and removal tools that are fairly safe. If one does encounter an issue, its always been damage done by the MALWARE. The most common issue being Layered Service Provider replacement which you can use LSPFIX (Hint: use http://www.google.com to find).
The thread has follow on discussions about new links if you don't want to use Google to find an item.
Bob
PS. All the tools are free.
When my wife first loaded Windows 2000 (previously having NT4) I thought the computer screamed, especially for only being a 500MHz computer. Several months later, the startup is so slow I can go watch a tv program (joke), but it is extrememly slow. Also, selecting any program or going from one page on the internet to another in IE is slow - heck, the whole computer is slow.
I suspect it is a compliation of things, from many programs being installed/uninstalled (making the residual registery big), too many programs starting up, and VPN usage for software development and testing.
I've run Norton, defraged, you know all the usually stuff, even tried a few registery cleaners - it's still slow.
Before we "nuke and repave", does anyone have any suggestions on fixing this situation? I mean besides buying a faster computer. (which is in the works, I'm looking for a newer replacement mother board - if you got recomendations on that, I'll take it too - ATX type box - Gateway GP7-500)
Thanks guys and gals~

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic