Recently my laptop has started becoming terribly unresponsive. Occasionally it will take ages to open anything. I've traced the problem to the hard drive. The hard drive itself is fine, but something was just writing and writing and writing. I found out it's explorer, lsass.exe, and csrss.exe doing this. Terminating explorer solves the problem instantly with no apparent ill effects.

So what the hell is explorer DOING?

Do you have Vista? Is it Superfetching?

Do you have more than one instance of lsass open? If lsass is exhibiting this sort of behavior I'd say it's definitely been replaced by a virus. csrss also generally shouldn't be doing this.

Probably the only reason I can think of for explorer.exe to stall out would be if it's in rundll.

You mean other the GUI disappearing? Because if it doesn't that's not the real explorer.exe, that's malware.

It's the real explorer, though the auto-restarting behavior explorer used to exhibit has also vanished. I have to re-start it myself. Yes it's XP.

lsass appears to be constantly writing even when the system isn't locking up.

Get Process Explorer and see exactly what the hell lsass is doing, and if there's a service under it causing the problem.

Conficker C spreads using a vulnerability in CSRSS