I've been wondering why things are suddenly feeling so slow. I checked it out, and apparently Tortoise SVN is the culprut. I need it to access and update various svn repositories, so can't remove it. Thankfully it's an easy fix - Travis over at Paraesthesia has the details.
The short version is to set TSVNCache.exe to scan only folders that actually contain svn checkouts, rather than all fixed disks, which is the default.