I went to run a defrag in Vista last night (via Defraggler), and noticed that the most-fragmented files were hundreds of files, each hundreds of megs, in C:/ProgramData/ Microsoft/ Windows/ WER/ ReportQueue. A bit of poking around revealed that they don’t serve much of a purpose.
Rather than emptying the folder by hand (which is probably safe from what I’ve seen), you can use the Disk Cleanup tool (included with Vista, type “Disk Cleanup” in the start menu’s textbox and it’ll come up) to do it. I reclaimed 7 GB of space, which is a lot when you’re talking about a tiny ~55GB partition for Vista. In researching this online, I found a few people talking about their ReportQueue folders growing to hundreds of gigs in size.
CCleaner hadn’t detected this folder, though I’m not running the latest version, and I certainly don’t mind it erring on the size of conservatism when finding system-created folders that it thinks I don’t need.