What could be making a seemingly clean Win XP slow?

It's an old install, but I've made it so there are hardly any services running, no viruses, no spyware, chkdsk'd defrag. Take everything out of msconfig. Everything looks like it was when I newly formatted it, but its just slow.

Now I know if I format it, it will be fast again. It's a mystery, I'm wondering if anyone knows anything else that you can do to make it like a new format without formatting?

try running CCleaner on it. It may be bogged down with a lot of other crap you never thought of.

Also, you said this was an old install, how old? Like pre-SP2 old? the reason I'm asking is that when XP first came out it didn't need as much RAM to run as it does now. So 256MB might have been fine pre-SP2, but you probably need closer to 512 for SP2 to run as smoothly.

Reformat will make it fast again most likely because XP like other windowses gets corrupted with time when people install programs on it. The inherent pitfall being system libraries and programs replacing them with other versions.

So the more programs you install to XP the more likely it is that there will be conflicts and resulting slowness. Only a complete reinstall will fix the libraries back to standard and return the speed.

I second the CCleaner option, but after you remove all junk files, also run the built-in registry cleaner several times, and rebot computer after, dont forget to defrag with JKDefrag

corrupt hard drive
failing memory
other failing hardware

network contention (it is trying to find a network resource that no longer exists)

I had the same problems.
2 months later my hard drive died.

I would suggest you to make a small partition and install XP and see what happens.
or check your harddrive, memory etc. with software.