NOTE: When adding useful contributons to this thread, please just batch a few programs into one single post, so that future referals to a certain program mentioned in the thread are easier to follow.
Also please ONLY add freeware programs!
For starters, two antivirus programs which are free and which maintain online databases which are constantly updated and which you can freely download and install:
You all should know this one, by now. it kills off spyware and other nasty little things that lurk on your machine.
http://www.lavasoftusa.com/software/adaware/
Yet another spyware cleaner, to catch the things that Ad-aware
misses.
http://www.safer-networking.org/index.php?page=download
The windows registry can get ridiculously messy after several months of software installs/uninstalls, but this program can clean out all the registry junk quite nicely.
http://www.majorgeeks.com/download.php?det=2048
A system cleaner which includes a registry cleaner, internet cleaner, history cleaner, temporary files cleaner, disk cleaner, desktop cleaner and start menu cleaner. You can even create your own cleaner. BeClean is freeware, and requires a reboot after installation.
http://boozet.xepher.net/beclean/
Let you monitor programs accessing the registry or any files.
http://www.sysinternals.com/ntw2k/source/regmon.shtml
http://www.sysinternals.com/ntw2k/source/filemon.shtml
Is that BeClean fully safe? Do you use it?
I don't use it, I just got it recommended at copied the recommendation as it is. Until somebody here uses it and gives credible feedback, the old rule still is in effect: No program that you give access to your system is 100% trustworthy if you do not know the source code.