This morning I was greeted to an especially unresponsive machine. The hard disk was thrashing away and I was pretty much not getting a usable machine for about 10-15 minutes as even typing became too slow for the machine.
While I was looking for the culprit of the sudden perf drop I was rummaging around the system making sure that defrag had run recently. I also checked disk space (plenty) and stumbled into the Disk Cleanup tool.
One odd thing that I ran into there: Check out the insane Windows Error Reporting sizes:
Ok I admit I've never checked or enabled/set any setting in Windows Error Reporting (Control Panel | Problem Reports and Solutions), but it seems truly insane of Windows to collect all of this data and NOT notify you that a) you have so much of it and b) that you might want to send it to Microsoft. Not that the latter will have much of an effect - I remember on my old machine I was probably generating 2 gig of this data a week given how frequently my video card kept crashing <g>.
Eventually I selected all of the problem reports and sent them to Microsoft - all 400 of them and 50 megs worth. When done and rerunning the disk cleanup tool viewer I ended up with much more reasonable numbers - about 50 megs worth of data on disk, which is presumably those requests that have not been resolved or have no flagged solutions. I deleted those too - screw it.

What's interesting here too is that my machine actually is fairly stable looking at the performance and reliability monitor I see very few crashes:
Overall Vista runs very stable for me. There are very few actual application failures and most of the ones on the list are actually from debugging applications in VS.NET that are crashing during debug. The few Windows failures are Explorer lockups of which there are very few now too that I've stopped using Explorer for file access and using Explorer 2 instead.
So the actual cause of the horrible performance this morning turned out to be bad timing - my scheduled defragging was taking place today (unbeknownst to me). It's scheduled for 3am in the morning but my machine was (surprisingly actually <s>) off this morning so it caught me on startup today. I also opted to download Office updates from Windows update at the same time - the Office update was writing a huge amount of data and ngen'ing its .NET assemblies which is always massively disk intensive. Between the two the disk was completely clogged and chugging down
Everything's back to normal now, but this is frustrating as heck. Things are bad enough most mornings when the machine is powering up from hibernate as all sorts of backed up tasks fire up on startup...