I know that Microsoft is considered an evil empire and shouldn't have any place on a web site devoted to Open Source and Freeware, but let's face it, internal combustion is widely used although there are viable alternatives available. Most of the end users I support can't use Linux or OS X because nobody developes the kind of applications they need (office management systems and medical records systems). So, here's some essential Windows security freeware:
This was MS Antispyware. I think I liked the old version better - it seemed a bit more techie, and much more stable. Beta 2 works fine on my laptop computer, but my dekstop gags on it. Admittedly, my laptop is 3 years newer than my dekstop, but they are both running WinXP and the desktop is typically very stable the rest of the time.
This antivirus app is for those of you who aren't computing in a networked, managed environment. Also, it is available for Windows XP and for Linux! Maybe they'll port it over to OS X soon - hint, hint. The pay version has a very nifty central management console (AVGAdmin) that has uses way beyond the typical AV console - such as figuring out which users were using which workstations when.
Windows OneCare Live
Free (for now), and soon to be an incredible deal (3 PC's for $49.95 per year). Thumbs up to the ease of use and comprehensive coverage, thumbs down to the draconian firewall and poor interface and manageability.
A good Microsoft-developed registry cleaner that seems to still work (for the most part, read on, gentle reader), despite the fact that it was made for Windows 95, 98, and NT. I know it still works well in Win2k and XP SP1. XP SP2 had problems and would not run it.
It is best for fixing registry bloat - that is, removing invalid or unnecessary entries. This can speed boots and generally speeds things up. I've also had improvements in stability.