Is it Limewire?
I can’t tell anything from the task manager, but my hard drive has been crunching serious bits for about a half hour now. So much so, that I have a hard time doing anything else. In fact typing this is taking forever.
My CPU is like 94% idle, but Limewire is poaching about 40 Meg, the lions share at present. Make it stop. Please. (How?)
Did you try to reboot?
I’m guessing you’re in I.T.? 
ETA: It’s stopped now, but I don’t see any appreciable difference in programs running in the task manager (CPU or Memory-wise). So why the fuck did my machine decide to go on a clandestine disk writing journey without my knowledge or consent?
I know only a little about Limewire, but I know that as long as you have Limewire running & have not disabled the function, you may be donating bandwidth to others w/ slower connections. (see:“ultrapeer”.)
You may also have been hosting some uploads; i.e., someone else’s download from you. I have noticed that this can slow down my Limewire considerably; although I can’t say that I’ve noticed it slowing down my system.
I don’t know if that’s any help.
P.S. Greyhound on espresso… <shudder>… scary imagery, there.
Maybe something went screwy with a Windows Service or something else that doesn’t show up in the Task Manager? Hard to say. When errors like that happen and then correct themselves, or something weird happens once and a reboot fixes it, I don’t usually spend too much time wondering about it.
Windows Update applied a lot of patches on my system overnight.
There are several things that might do that:
Full drive indexing (Vista, Google Desktop, etc.) when cataloging the drive can take a dis-proportionate amount of resources
Virus scanning - My personal gripe…something that is supposed to keep you safe from resource sucking viruses can make your machine unuseable while looking for those resource sucking viruses.
Backup, defrag, disk health tools - all can slow down the rest of the system while they’re running.
Limewire? Perhaps. But usually the bottleneck there is your bandwidth to the ISP.
Virus/Malware/beasties - very possible, but without knowing what you have for software protection, what OS you have (98, XP, Vista ultimate Pro Turbo ++), and the usage patterns you have (Web and email only, Video editing, Every Pop-up that says I’m infected gets clicked), I can’t begin to guess.
If you add the I/O Reads and/or I/O Writes columns to Task Manager’s default view, it might give you an idea of which process is using the disk so much. I believe Vista’s Task Manager may have more available counters. Or you could use one of the Sysinternals utilities like Filemon.
I had this going frequently about a year ago, and spent a nightmarish couple of days reinstalling my entire system and purchasing Spyware Detector. Since then, I’ve had very little thrashing. Of course, I’m not on a LAN, but I wasn’t before either.
Another possibility is that it’s creating a System Restore point.
Maybe somebody else in your house signed up for the SETI Project?