Agent Ransack - Much better than the standard windows (7) search facilities, however, even for me, several months can pass by between two uses of this.
CCleaner - Helps keep your computer running smoothly. I typically run this about once every other month.
Dependency Walker - Explore windows dll files. I remember this once being an absolute godsend, however I rarely use it anymore.
Greenfish - Pretty decent icon editor.
HTML Help Workshop From Microsoft - just the command line hhc part (Htmlhelp.exe), needed to rebuild help (.chm) files.
mini.chm - My repackage of the MSDN windows API reference (48MB), which integrates quite nicely with Edita.
OllyDbg - 32 bit windows debugger. This and/or FDBG a must for debugging low-level inline assembly such as the Phix runtime.
Process Monitor - See what programs are really getting up to, in terms of file i/o, group policy permissions, registry and network access, etc. Historically, I have needed this almost daily when coding in C++, a bit when coding in assembly (occasionally including the back end parts of Phix), and almost never when coding in (hll) Phix. Actually there was a fairly recent need: monitoring attempts to load the IUP dlls (that I had just downloaded) proved to be extremely helpful. On Linux you can try running
strace -s99999 -efile command_nameto obtain similar details.
Despite recent problems, I am still quite the fan of Avast and Opera, but hand on heart I cannot really recommend either anymore. To be fair, Avast has recently turned the corner and is now slowly regaining my trust, but it still has a little further to go yet.